[18-8-stable] Remove release instance deployment trigger from Ubuntu-20.04-staging job

What does this MR do and why?

This MR backports !9126 (merged) to 18-8-stable, which restricts Ubuntu-20.04-staging deployment to RC tags only. This prevents deployment to the deprecated release.gitlab.com for final release tags.

Original MR: !9126 (merged)
Related issue: gitlab-com/gl-infra/delivery#21084 (closed)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The original MR has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).