Geo: Deprecate support for legacy storage

• Docs change: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8133
• Geo status task checks for hashed storage => https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8739
• If Geo is enabled with legacy storage, show a warning on admin Geo nodes page => https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8433
• Add deprecation notice on the release post: "With 11.6, Geo no longer supports legacy storage. After 11.6, Geo may not work properly without hashed storage enabled." See the release post gitlab-com/www-gitlab-com!17025 (merged)
• Open issue for the 12.0 release => https://gitlab.com/gitlab-org/gitlab-ee/issues/8690