The OLD behaviors of all policies are deprecated, but only by
documentation. Add an explicit deprecation diagnostic for policies
introduced in CMake 3.15 and below to encourage projects to port
away from setting policies to OLD.
We do not add default warning flags on other compilers, and having
a warning flag in the default flags makes it hard for projects to
customize the warning level. They need to use string processing
to remove `/W3` from `CMAKE_{C,CXX}_FLAGS`. Therefore we should
drop it.
However, projects may be using string processing to replace `/W3`
with another flag, so we cannot simply drop it. Add a policy to
drop it in a compatible way.
Fixes: #18317
