Help: Document cmake_minimum_required deprecation of old versions

Document the deprecation warnings added by commit 5845c218 (Deprecate
compatibility with CMake versions older than 2.8.12, 2020-06-12,
v3.19.0-rc1~629^2) and commit 3a479154 (Deprecate compatibility with
CMake versions older than 3.5, 2023-02-09, v3.27.0-rc1~508^2).

Fixes: #25196

Help/command/DEPRECATED_POLICY_VERSIONS.txt

+.. versionchanged:: 3.27
+
+  Compatibility with versions of CMake older than 3.5 is deprecated.
+  Calls to :command:`cmake_minimum_required(VERSION)` or
+  :command:`cmake_policy(VERSION)` that do not specify at least
+  3.5 as their policy version (optionally via ``...<max>``)
+  will produce a deprecation warning in CMake 3.27 and above.
+
+.. versionchanged:: 3.19
+
+  Compatibility with versions of CMake older than 2.8.12 is deprecated.
+  Calls to :command:`cmake_minimum_required(VERSION)` or
+  :command:`cmake_policy(VERSION)` that do not specify at least
+  2.8.12 as their policy version (optionally via ``...<max>``)
+  will produce a deprecation warning in CMake 3.19 and above.

Help/command/cmake_minimum_required.rst

@@ -80,6 +80,8 @@ invokes
 
 which enables compatibility features for CMake 2.4 and lower.
 
+.. include:: DEPRECATED_POLICY_VERSIONS.txt
+
 See Also
 ^^^^^^^^
 

Help/command/cmake_policy.rst

@@ -52,6 +52,8 @@ version and tells newer CMake versions to warn about their new policies.
 Note that the :command:`cmake_minimum_required(VERSION)`
 command implicitly calls ``cmake_policy(VERSION)`` too.
 
+.. include:: DEPRECATED_POLICY_VERSIONS.txt
+
 Setting Policies Explicitly
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^