Remove const from iterator operator return types

Commit f28fc22 introduced const qualifiers on post-(inc-/dec-)rement
operators of iterators. These qualifiers prevent the use of basic_json
in place of std::ranges::range, which requires the post-increment
operator to be equality-preserving.

These changes appear to be the result of compiler suggestions, and no
further explanation is discernible from the PR discussion (nlohmann#858).

This commit partially reverts f28fc22, removing all added const
qualifiers.

Fixes nlohmann#3331.

include/nlohmann/detail/iterators/iter_impl.hpp

@@ -352,7 +352,7 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
     @brief post-increment (it++)
     @pre The iterator is initialized; i.e. `m_object != nullptr`.
     */
-    iter_impl const operator++(int) // NOLINT(readability-const-return-type)
+    iter_impl operator++(int)
     {
         auto result = *this;
         ++(*this);
@@ -403,7 +403,7 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
     @brief post-decrement (it--)
     @pre The iterator is initialized; i.e. `m_object != nullptr`.
     */
-    iter_impl const operator--(int) // NOLINT(readability-const-return-type)
+    iter_impl operator--(int)
     {
         auto result = *this;
         --(*this);

include/nlohmann/detail/iterators/json_reverse_iterator.hpp

@@ -48,7 +48,7 @@ class json_reverse_iterator : public std::reverse_iterator<Base>
     explicit json_reverse_iterator(const base_iterator& it) noexcept : base_iterator(it) {}
 
     /// post-increment (it++)
-    json_reverse_iterator const operator++(int) // NOLINT(readability-const-return-type)
+    json_reverse_iterator operator++(int)
     {
         return static_cast<json_reverse_iterator>(base_iterator::operator++(1));
     }
@@ -60,7 +60,7 @@ class json_reverse_iterator : public std::reverse_iterator<Base>
     }
 
     /// post-decrement (it--)
-    json_reverse_iterator const operator--(int) // NOLINT(readability-const-return-type)
+    json_reverse_iterator operator--(int)
     {
         return static_cast<json_reverse_iterator>(base_iterator::operator--(1));
     }

include/nlohmann/detail/iterators/primitive_iterator.hpp

@@ -87,7 +87,7 @@ class primitive_iterator_t
         return *this;
     }
 
-    primitive_iterator_t const operator++(int) noexcept // NOLINT(readability-const-return-type)
+    primitive_iterator_t operator++(int) noexcept
     {
         auto result = *this;
         ++m_it;
@@ -100,7 +100,7 @@ class primitive_iterator_t
         return *this;
     }
 
-    primitive_iterator_t const operator--(int) noexcept // NOLINT(readability-const-return-type)
+    primitive_iterator_t operator--(int) noexcept
     {
         auto result = *this;
         --m_it;

single_include/nlohmann/json.hpp

@@ -11308,7 +11308,7 @@ class primitive_iterator_t
         return *this;
     }
 
-    primitive_iterator_t const operator++(int) noexcept // NOLINT(readability-const-return-type)
+    primitive_iterator_t operator++(int) noexcept
     {
         auto result = *this;
         ++m_it;
@@ -11321,7 +11321,7 @@ class primitive_iterator_t
         return *this;
     }
 
-    primitive_iterator_t const operator--(int) noexcept // NOLINT(readability-const-return-type)
+    primitive_iterator_t operator--(int) noexcept
     {
         auto result = *this;
         --m_it;
@@ -11728,7 +11728,7 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
     @brief post-increment (it++)
     @pre The iterator is initialized; i.e. `m_object != nullptr`.
     */
-    iter_impl const operator++(int) // NOLINT(readability-const-return-type)
+    iter_impl operator++(int)
     {
         auto result = *this;
         ++(*this);
@@ -11779,7 +11779,7 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
     @brief post-decrement (it--)
     @pre The iterator is initialized; i.e. `m_object != nullptr`.
     */
-    iter_impl const operator--(int) // NOLINT(readability-const-return-type)
+    iter_impl operator--(int)
     {
         auto result = *this;
         --(*this);
@@ -12167,7 +12167,7 @@ class json_reverse_iterator : public std::reverse_iterator<Base>
     explicit json_reverse_iterator(const base_iterator& it) noexcept : base_iterator(it) {}
 
     /// post-increment (it++)
-    json_reverse_iterator const operator++(int) // NOLINT(readability-const-return-type)
+    json_reverse_iterator operator++(int)
     {
         return static_cast<json_reverse_iterator>(base_iterator::operator++(1));
     }
@@ -12179,7 +12179,7 @@ class json_reverse_iterator : public std::reverse_iterator<Base>
     }
 
     /// post-decrement (it--)
-    json_reverse_iterator const operator--(int) // NOLINT(readability-const-return-type)
+    json_reverse_iterator operator--(int)
     {
         return static_cast<json_reverse_iterator>(base_iterator::operator--(1));
     }