Document the deprecation warnings added by commit 5845c218d7 (Deprecate
compatibility with CMake versions older than 2.8.12, 2020-06-12,
v3.19.0-rc1~629^2) and commit 3a4791548d (Deprecate compatibility with
CMake versions older than 3.5, 2023-02-09, v3.27.0-rc1~508^2).
Fixes: #25196