Merge branch 'update-security-txt' into 'master'

Bump the expiry, update thanks, and describe format of security.txt

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/138130



Merged-by: Patrick Cyiza <jpcyiza@gitlab.com>
Approved-by: Patrick Cyiza <jpcyiza@gitlab.com>
Co-authored-by: Nick Malcolm <nmalcolm@gitlab.com>