Mark the legacy _NameMap initializer as deprecated. (#1816)

No one calls these directly, but the old generated code code. This will
hopefully nudge folks to regenerate their sources to take advantage of
the newer generation that comes from smaller codegen and has some better
memory overhead during startup.

Author: thomasvl

Sources/SwiftProtobuf/NameMap.swift

@@ -304,6 +304,11 @@ public struct _NameMap: ExpressibleByDictionaryLiteral {
     #else  // !REMOVE_LEGACY_NAMEMAP_INITIALIZERS
 
     /// Build the bidirectional maps between numbers and proto/JSON names.
+    @available(
+        *,
+        deprecated,
+        message: "Please regenerate your .pb.swift files with the current version of the SwiftProtobuf protoc plugin."
+    )
     public init(
         reservedNames: [String],
         reservedRanges: [Range<Int32>],
@@ -316,6 +321,11 @@ public struct _NameMap: ExpressibleByDictionaryLiteral {
     }
 
     /// Build the bidirectional maps between numbers and proto/JSON names.
+    @available(
+        *,
+        deprecated,
+        message: "Please regenerate your .pb.swift files with the current version of the SwiftProtobuf protoc plugin."
+    )
     public init(dictionaryLiteral elements: (Int, NameDescription)...) {
         initHelper(elements)
     }