[TECH-228] Remnants of TECH-224

[asimmon]

• Fixed: Rule GMDTR09 "Handlers should not call other handlers" was not applied when using our own new "Async"-suffixed extension methods, added more tests
• Do what’s Microsoft is doing: Using ImmutableHashSet<T> in analyzers instead of HashSet<T>
• Ignore more Renovate dependencies update. We don’t want to upgrade some packages (Microsoft.CodeAnalysis.* and BCL async interfaces) because we want to set a minimal required dependency