Add GHES 3.18 to GHAS feature matrix

[felickz]

Update the matrix: https://github.com/advanced-security/advanced-security-material/blob/main/GHAS-on-GHES-feature-matrix.md

Highlight major GHAS security feature updates from the release notes: https://docs.github.com/en/enterprise-server@3.18/admin/release-notes

• End of life dates at: https://docs.github.com/en/enterprise-server@3.18/admin/all-releases#releases-of-github-enterprise-server