Code cleanup: Phase 1

[NickCraver]

Tidying up for newer C# versions and removing old cruft:

• Removing unneeded pragmas (e.g. transitive obsoletes)
• Switch expressions
• Expression body usages, where it's cleaner with new switch expressions
• Statics where useful to remove virtual calls
• A handful of pattern matching usages where we were repeat casting
• StringBuilder optimizations for .NET Core (single char appends)
• Suppression fixes for contention between frameworks
• Suppression additions for dupe enum values (that are there for compat)
• Fix messages/arguments for many argument exceptions
• Re-enables the GC test (was the only skipped one - no longer needed)
• Misc compiler warning fixes
• Fixes KestrelRedisServer's conflict with System.Runtime.CompilerServices.Unsafe

These are broken up by commit for an easier review, but didn't want to make 12 PRs for some maintenance catch-up.

Note: this does not fix all informational message, including a few suspects for actual issues and some that are the way we structure Dispose() => Dispose(true) in a few files. Given those may change functionality, will do a follow-up PR there. This is meant to get all the noise gone and general cleanup without any significant impact anywhere.