Help: Add boilerplate text to use for removed policies

Help/policy/REMOVED_EPILOGUE.txt

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+This policy was introduced in CMake version |INTRODUCED_IN_CMAKE_VERSION|.
+Prior to removal in CMake version |REMOVED_IN_CMAKE_VERSION|, it could be
+set by :command:`cmake_policy` or :command:`cmake_minimum_required`.
+If it was not set, CMake |WARNED_OR_DID_NOT_WARN|, and used ``OLD`` behavior.

Help/policy/REMOVED_PROLOGUE.txt

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+.. warning::
+  The ``OLD`` behavior of this policy was removed
+  in CMake version |REMOVED_IN_CMAKE_VERSION|.
+  This policy must be set to ``NEW`` by a call to
+  :command:`cmake_minimum_required` or :command:`cmake_policy`.