Merge topic 'post-rel-dev'

2c209097b9 Configure CMake itself with policies through CMake 3.20
83a3db5631 export: Increase maximum policy version in exported files to 3.20
d76cb12905 Add deprecation warnings for policies CMP0088 and below

Acked-by: Kitware Robot <kwrobot@kitware.com>
Merge-request: !6290