sp_BlitzWho: drop pre-2016 support and modernize code (#3874)

Following the same pattern as sp_Blitz (#3872) and sp_BlitzCache (#3870):

- Add version gate before procedure creation (reject pre-2016, bypass Azure)
- Add standard session settings header
- Delete entire SQL 2008 R2/2010 code branch (~200 lines)
- Remove DBCC INPUTBUFFER cursor fallback (~55 lines)
- Remove @EnhanceFlag, @SessionWaits, @ProductVersionMinor variables
- Strip always-true version guards from output column mapping
- Simplify memory grant hint to just memory threshold check
- Replace IF OBJECT_ID...DROP with DROP TABLE IF EXISTS
- Update help text to reflect 2016+ minimum

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: BrentOzar