Update GHAS feature matrix for GHES 3.20

Add GHES 3.20 column (released 2026-03-17, deprecation 2027-03-17) to all
feature tables. CodeQL toolcache version: 2.23.9.

New features added:
- Secret Protection: validity checks, alert assignees, enterprise-level
  delegated bypass for push protection, partner pattern count 478
- Code Scanning: CodeQL Rust GA (was Preview in 3.19), C/C++ without-build GA,
  code scanning alert assignees (Preview)
- Dependabot Updates: Conda package support, custom runner labels
- Administration: Enterprise Security Manager role (Preview)

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

Author: github-actions[bot]