Remove PackageCloud upload machinery from omnibus-gitlab
## Summary
Remove all PackageCloud package upload/CI machinery from omnibus-gitlab. PackageCloud (`packages.gitlab.com`) has been fully superseded by Pulp for package distribution. The `PACKAGECLOUD_ENABLED` variable has been set to `"false"` (disabled) for some time. This issue tracks the complete removal across 7 sequential MRs.
## Background
omnibus-gitlab uses a `PackageRepository` abstraction to upload built packages to a distribution repository. The old implementation (`PackageCloudRepository`) uploaded to `packages.gitlab.com` via the `package_cloud` CLI gem. This has been replaced by `PulpRepository` which uploads to `pulp.gitlab.com`. The PackageCloud path is now fully dead code.
## Scope
The removal is split into 7 MRs that must be merged in order:
| MR | Branch | Content |
|----|--------|---------|
| [MR1](https://gitlab.com/gitlab-org/omnibus-gitlab/-/merge_requests/9288) :white_check_mark: | `remove-packagecloud-ci` | Remove PackageCloud CI templates, \~62 staging/release jobs, and `PACKAGECLOUD_ENABLED` variable |
| [MR2](https://gitlab.com/gitlab-org/omnibus-gitlab/-/merge_requests/9311) :white_check_mark: | `dattang/rename-pulp-jobs` | Rename `-pulp` CI jobs to drop the suffix (e.g. `Ubuntu-20.04-staging-pulp` -\> `Ubuntu-20.04-staging`); flatten upload/release template pairs into single templates |
| [MR3](https://gitlab.com/gitlab-org/omnibus-gitlab/-/merge_requests/9337):white_check_mark: | `dattang/remove-packagecloud-scripts` | Remove `INCLUDE_PACKAGECLOUD` guard from `prepare_bundle.sh` and PackageCloud fallback from `install_package.sh` |
| [MR4](https://gitlab.com/gitlab-org/omnibus-gitlab/-/merge_requests/9344) | `dattang/remove-packagecloud-gem` | Remove `package_cloud` gem group from `Gemfile` |
| MR5 | `dattang/remove-packagecloud-ruby` | Delete `PackageCloudRepository` class, simplify `PackageRepository` factory to always use Pulp, remove `REPOSITORY_TYPE` variable |
| MR6 | `dattang/remove-packagecloud-docker` | Remove `PACKAGECLOUD_REPO` unstable check from Docker `init-container` |
| MR7 | `dattang/remove-packagecloud-docs` | Update all documentation and comments |
## What is NOT removed
- `source 'https://packagecloud.io/cinc-project/stable'` gem source - this is Chef/Cinc gem hosting, unrelated to package uploads
- `.raspbian_release_template` and Raspberry Pi release jobs - separate decision\]
issue