The OLD behaviors of all policies are deprecated, but only by
documentation. Add an explicit deprecation diagnostic for policies
introduced in CMake 3.24 and below to encourage projects to port
away from setting policies to OLD.
When running CMake for the first time in a build tree, for some
generators CMake would set compiler environment variables
like CC, CXX, etc. when the corresponding language is enabled.
That behavior was never documented and can result in different
behavior between the first and subsequent runs. Add a policy
to no longer set those environment variables.
Fixes: #21378
Brad King