CompatibilityLevel for introspection

> If `RepeatableDirectives` and/or other new features are going to break introspection queries, it is likely that some users will want to maintain compatibility with older versions regardless of what is the newest release. Perhaps we should add a property to the schema called something like `CompatibilityLevel`, which could be set to `June2018`, `October2021`, `WorkingDraft` and `Experimental`.

_Originally posted by @Shane32 in https://github.com/graphql-dotnet/graphql-dotnet/issues/3570#issuecomment-1494431372_