Add compat note for 7.1 (#5011)

Author: comandeo-mongo

source/reference/compatibility.txt

@@ -98,7 +98,7 @@ is deprecated.
      - |checkmark|
 
    * - 7.1
-     -
+     - |checkmark| [#mongoid-7.1-ruby-3.0]_
      - |checkmark|
      - |checkmark|
      - |checkmark|
@@ -131,6 +131,8 @@ is deprecated.
 
 .. [#mongoid-7.2-ruby-3.0] Mongoid version 7.2.4 or higher is required.
 
+.. [#mongoid-7.1-ruby-3.0] Mongoid version 7.1.9 or higher is required.
+
 .. [#ruby-2.4] Ruby version 2.4.1 or higher is required.
 
 .. [#ruby-2.2] Ruby version 2.2.2 or higher is required.