Enable Tiered Compilation by default (dotnet/coreclr#19525)

Enable Tiered Compilation by default (dotnet/coreclr#19525)

Enable Tiered Compilation by default

1) Changes the default state of the tiered compilation feature check to be ON BY DEFAULT
2) Removed comments about the source about this being a work in progress. Although it will surely continue to evolve and improve, remaining issues would be better tracked in our issue tracking system with the same default presumption as other runtime features - assume it works unless noted otherwise.
3) Adjusts a number of tests and automated scripts that made assumptions that the default setting of this feature is off.
4) Stop accepting the deprecated env var COMPLUS_EXPERIMENTAL_TieredCompilation. I'm not aware it has any remaining usage but if so we're going to find out.
5) Adjust config names for JitBench

Commit migrated from https://github.com/dotnet/coreclr/commit/f6174b93d100d46f4641f040b6de5fa254c1ee71