[clang-tidy] Deprecate cert-dcl21-cpp

[carlosgalvezp]

Summary

It is no longer part of the CERT standard. Looking at the
CERT webpage, we can see it has been moved to the Void
section:
https://wiki.sei.cmu.edu/confluence/display/cplusplus/5+The+Void

[Eugene.Zelenko]

I think will be good idea to track deprecated features on GitHub issues with something like deprecate in XYZ label(s).

[carlosgalvezp]

In D147563#4244227, @Eugene.Zelenko wrote:

I think will be good idea to track deprecated features on GitHub issues with something like deprecate in XYZ label(s).

Great idea, I started to have the feeling we have quite a few deprecated things to keep track off, will give some thought and do it!

clang-tools-extra/docs/ReleaseNotes.rst

 - Improved :doc:`bugprone-unchecked-optional-access
   <clang-tidy/checks/bugprone/unchecked-optional-access>` check to properly handle calls
   to ``std::forward``.
 
 - Improved :doc:`bugprone-use-after-move
   <clang-tidy/checks/bugprone/use-after-move>` check to also cover constructor
   initializers.
 
+- Deprecated :doc:`cert-dcl21-cpp
+  <clang-tidy/checks/cert/dcl21-cpp>` check.
+
 - Deprecated check-local options `HeaderFileExtensions`
   in :doc:`google-build-namespaces
   <clang-tidy/checks/google/build-namespaces>` check.
   Global options of the same name should be used instead.
 
 - Deprecated check-local options `HeaderFileExtensions`
   in :doc:`google-global-names-in-headers
   <clang-tidy/checks/google/global-names-in-headers>` check.

clang-tools-extra/docs/clang-tidy/checks/cert/dcl21-cpp.rst

 .. title:: clang-tidy - cert-dcl21-cpp
 
 cert-dcl21-cpp
 ==============
 
+.. note::
+  This check is deprecated since it's no longer part of the CERT standard.
+  It will be removed in :program:`clang-tidy` version 19.
+
 This check flags postfix ``operator++`` and ``operator--`` declarations
 if the return type is not a const object. This also warns if the return type
 is a reference type.
 
 The object returned by a postfix increment or decrement operator is supposed
 to be a snapshot of the object's value prior to modification. With such an
 implementation, any modifications made to the resulting object from calling
 operator++(int) would be modifying a temporary object. Thus, such an