Update CMake to require version 3.21

The current CMake files are set for 3.20 since this is the version that comes with VS 2019 (16.11). Moving to 3.21 as a minimum would clean up the projects and make them more 'modern'.

VS 2022 ships with 3.21 or later.

GitHub Actions windows-2019 runner images and MS-Hosted windows-2019 VM images have newer versions of CMake.

This would require installing a newer CMake for the Developer Command Prompt for VS 2019 if using CMake.

This would include:

• Use schema version 3 of CMakePresets.json which adds installDir and toolchainFile variables instead of having to set it through cache variables.

• Tests and subtests can use PROJECT_IS_TOP_LEVEL instead of the current workaround.

• For CMake tests, this also allows the use of TARGET_RUNTIME_DLLS without a guard or fallback.