Remove dismissal ability for manual banners

What does this MR do and why?

⚠ Depends on !76256 (merged) to be merged first.

We decided to remove the dismissal ability of the manual renewal and manual quarterly co-term banners in !75912 (comment 777396722). This merge request will remove the code that allowed the banners to be dismissable.

Another change included here, is the replacement of two css classes (gl-mt-5 gl-mb-5) with one (gl-my-5) for the manual renewal banner as it was mentioned in this comment.

Info: The banners are behind a feature flag and therefore do not need a changelog.

Screenshots or screen recordings

page	screenshot
/admin
/admin/license

How to set up and validate locally

• See !75912 (merged) for the manual renewal banner.
• See !76256 (merged) for the manual quarterly co-term banner.

MR acceptance checklist

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

• I have evaluated the MR acceptance checklist for this MR.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/346259 and https://gitlab.com/gitlab-org/gitlab/-/issues/346260