Merge version-specific changes update docs for all installation methods
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### Problem to solve
We have version-specific changes with each minor version that users need to follow when upgrading. Those are scattered in various pages and are hard to follow:
- Generic https://docs.gitlab.com/ee/update/#version-specific-upgrading-instructions
- Omnibus https://docs.gitlab.com/omnibus/update/#version-specific-changes
- Geo https://docs.gitlab.com/ee/administration/geo/replication/version_specific_updates.html
- Charts https://docs.gitlab.com/charts/releases/
### Proposal
Create specific version-specific pages which will include information based on the major version, like we now do for Omnibus (for example `gitlab_14_changes.md`).
- Then, we should merge the following specific info:
- https://docs.gitlab.com/ee/update/#version-specific-upgrading-instructions
- https://docs.gitlab.com/package/update/#version-specific-changes
- https://docs.gitlab.com/ee/administration/geo/replication/version_specific_updates.html
- Each doc will have an H2 heading with the minor release, and specific update changes for:
- Omnibus/Docker
- Source
- Geo
We'll need to think how Charts will fit into this. They have specific instructions as well, but their versioning is not the same as GitLab's https://docs.gitlab.com/charts/releases/.
### Who can address the issue
The TW team.
### Other links/references
- https://gitlab.com/gitlab-org/gitlab/-/issues/326346
- https://gitlab.com/gitlab-org/omnibus-gitlab/-/issues/6363
## Steps
1. [ ] Create a template doc for all changes (General, Package, Source, Geo, etc.)
1. [ ] Move Omnibus changes under the GitLab repository:
1. [ ] Move https://docs.gitlab.com/omnibus/update/gitlab_14_changes.html to `doc/update/gitlab_14_changes.md`
1. [ ] Move https://docs.gitlab.com/omnibus/update/gitlab_13_changes.html to `doc/update/gitlab_13_changes.md`
1. [ ] Move https://docs.gitlab.com/omnibus/update/gitlab_12_changes.html to `doc/update/gitlab_12_changes.md`
1. [ ] Move https://docs.gitlab.com/omnibus/update/gitlab_11_changes.html to `doc/update/gitlab_11_changes.md`
1. [ ] Move the versions sections that are shown under https://docs.gitlab.com/ee/update/#version-specific-upgrading-instructions to their respective location.
1. [ ] Update the section in https://docs.gitlab.com/ee/update/#version-specific-upgrading-instructions to include information from:
- https://docs.gitlab.com/ee/update/package/#version-specific-changes
- https://docs.gitlab.com/ee/administration/geo/replication/version_specific_updates.html
1. [ ] Rename https://docs.gitlab.com/ee/update/#version-specific-upgrading-instructions to https://docs.gitlab.com/ee/update/#version-specific-changes
1. [ ] Redirect https://docs.gitlab.com/ee/administration/geo/replication/version_specific_updates.html to https://docs.gitlab.com/ee/update/#version-specific-changes.
1. [ ] Update https://docs.gitlab.com/ee/update/package/#version-specific-changes to link back to https://docs.gitlab.com/ee/update/#version-specific-changes
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