sp_BlitzCache: drop pre-2016 support and modernize code (#3870)

Raise the minimum SQL Server version from 2008 to 2016, remove dead
compatibility branches, modernize syntax, and improve performance.

- Raise version floor to SQL Server 2016 in both gate checks
- Remove dead code for pre-2016 versions (~80 lines): NULL-substitution
  ELSE branches, trigger accuracy workaround, deprecated memory clerk
  columns (single_pages_kb), always-true version guards
- Replace 25 IF OBJECT_ID/DROP TABLE patterns with DROP TABLE IF EXISTS
- Cache repeated COUNT(*) on filter temp tables into variables
- Replace 4 WHILE loops for CSV hex parsing with STRING_SPLIT (~120 lines)
- Simplify function stats and row goals version checks

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

Author: BrentOzar