Extend commit eb35d8884b (find_package: Use PackageName_ROOT variables
as search prefixes, 2018-03-15, v3.12.0-rc1~349^2) to also check
upper-case `<PACKAGENAME>_ROOT` variables. Add policy `CMP0144` to
enable the behavior in a compatible way.
Fixes: #24403
Many environment variables were documented late and got
assigned wrong versions by the script.
(The whole Help/envvar section was only added in 3.10).
Issue: #19715
Add documentation for both the CMake variable and environment variable
of this name pattern. Update references to these names to link to their
documents. Clarify the pattern used to construct their names.