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 `cmStateSnapshot::RaiseScope` raises a variable in to a parent
directory scope, it uses `GetBuildsystemDirectoryParent` to find the
current top-most scope on the directory's stack. Since commit 3f4e5e8c3d
(cmState: Return end snapshot for GetBuildsystemDirectoryParent.,
2015-09-01, v3.4.0-rc1~100^2~1), that depends on the `DirectoryEnd`
field in the directory's state. However, when variable-only scopes were
added by commit 6954c8936f (cmState: Add a VariableScope snapshot type.,
2015-08-01, v3.4.0-rc1~179^2~1), we neglected to account for the
addition of that field by commit 52dbe654de (cmState: Record the end
position of each directory., 2015-08-01, v3.4.0-rc1~251^2~1).
Prior to commit 44a2f3f332 (Add new flow-control commands for variables
and policies scopes management, 2022-08-05, v3.25.0-rc1~257^2) this
problem went unnoticed because there was no way to have a variable scope
at the top of a directory's stack while processing a subdirectory. Now
the `block()/endblock()` commands enable the behavior, so fix tracking
of a variable scope as the top-most scope in a directory.
Fixes: #24138
Brad King