Since the `Visual Studio 18 2026` generator is new, we can switch
to the `.slnx` file format without changing behavior for users of
VS 2022 and older.
Fixes: #25887
The OLD behaviors of all policies are deprecated, but only by
documentation. Add an explicit deprecation diagnostic for policies
introduced in CMake 3.26 and below to encourage projects to port
away from setting policies to OLD.
For policy-specific tests, use the version before the policy was
introduced. Otherwise, use 3.5 where possible.
Also, remove `cmake_minimum_required()` and `project()` calls from
individual cases where they are handled by `CMakeLists.txt`.
Add a `VS_STARTUP_PROJECT` directory property to specify the project
that should be placed first in the `.sln` file so that it will be
selected as the default startup project.
Co-Author: Taylor Braun-Jones <taylor.braunjones@avigilon.com>
