Move upgrade path information to `gitlab-org/gitlab`

In #420831 (closed) we decided to move the upgrade path information from gitlab-com/support/toolbox/upgrade-path to gitlab-org/gitlab.

Acceptance Criteria

• upgrade-path.yml moved from gitlab-com/support/toolbox/upgrade-path to gitlab-org/gitlab.
• All MRs below have been merged.
• docs are introduced/updated when and how to update the upgrade path information.

Tracking related MR and merge status

• gitlab-org/gitlab: Add upgrade_path.yml to control required stops (!154783 - merged)
• gitlab-com/support/toolbox/upgrade-path: Move upgrade-path.yml location (gitlab-com/support/toolbox/upgrade-path!43 - merged)
• gitlab-org/release-tools: Move upgrade-path.yml location (release-tools!3148 - merged)
• gitlab-org/quality/upgrade-tester: https://gitlab.com/gitlab-org/quality/upgrade-tester/-/merge_requests/22+
• gitlab-org/gitlab-qa: Move upgrade-path location (gitlab-qa!1326 - merged)
• gitlab-org/quality/pg-dump-generator: Move upgrade-path.yml location (gitlab-org/quality/pg-dump-generator!31 - merged)