Stop uploading Omnibus packages to PackageCloud

What does this MR do?

Add PACKAGECLOUD_ENABLED gate and decouple pulp templates from packagecloud-dependent templates.

As we are decommissioning PackageCloud (PC), make the PC jobs optional so we can test and rollback if needed with only an env var change.

• Add .skip_unless_packagecloud_enabled rule to skip jobs when PACKAGECLOUD_ENABLED is not true
• Apply the rule to all jobs uploading to packagecloud
• Refactor staging upload templates to separate packagecloud and pulp concerns:
  • Introduce .staging_upload_template as a shared base (common settings: stage, image, script, cache, retry, tags)
  • Introduce .packagecloud_staging_upload_template extending the base, with INCLUDE_PACKAGECLOUD: "true" and the packagecloud-specific rules
  • Refactor .pulp_staging_upload_template to extend the base instead of being standalone
  • Rename derived packagecloud templates to carry the packagecloud_ prefix:
    • .fips_staging_upload_template → .packagecloud_fips_staging_upload_template
    • .staging_upload_with_rc_deployment_template → .packagecloud_staging_upload_with_rc_deployment_template
  • This makes it clear which templates are packagecloud-specific, and prepares for their removal in a follow-up MR

Related issues

Ref: https://gitlab.com/gitlab-org/build/team-tasks/-/work_items/177

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• MR title and description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• Latest Merge Result pipeline is green.
• When ready for review, MR is labeled workflowready for review per the Distribution MR workflow.

For GitLab team members

If you don't have access to this, the reviewer should trigger these jobs for you during the review process.

• The manual Trigger:ee-package jobs have a green pipeline running against latest commit.
• If config/software or config/patches directories are changed, make sure the build-package-on-all-os job within the Trigger:ee-package downstream pipeline succeeded.
• If you are changing anything SSL related, then the Trigger:package:fips manual job within the Trigger:ee-package downstream pipeline must succeed.
• If CI configuration is changed, the branch must be pushed to dev.gitlab.org to confirm regular branch builds aren't broken.

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes.
• Documentation created/updated.
• Tests added.
• Integration tests added to GitLab QA.
• Equivalent MR/issue for the GitLab Chart opened.
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.