Update GHAS feature matrix for GHES 3.20

- Add GHES 3.20 column to all feature tables (released 2026-03-17)
- Archive GHES 3.12 and 3.13 columns (both past deprecation date)
- Update CodeQL toolcache to 2.23.9
- CodeQL Rust language support promoted from Preview (☑️) to GA (✅)
- Add new feature rows:
  - Secret scanning: alert assignees (✅)
  - Code scanning: C/C++ without a build GA (✅)
  - Code scanning: alert assignees preview (☑️)
  - Dependabot: Conda package support (✅)
  - Dependabot: custom runner labels for self-hosted runners (✅)
  - Administration: Enterprise Security Manager role enterprise-level (☑️)
  - Administration: code scanning default setup bypass of Actions policies (✅)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]