Show update branch message when MR branch diverged from target branch

What does this MR do?

Show update branch message when MR branch diverged from target branch

Previously, the staleness message shown for the security report on an MR was determined using this (pseudo-code) logic:

if target_branch_security_report_is_older_than_7_days:
  if mr_branch_diverged_from_target_branch:
    render "Update your branch with latest changes from target branch"
  else:
    render "Run a new pipeline for target branch"

This change un-nests the logic so that each condition (whether the target branch's latest security report is older than 7 days; whether the MR branch has diverged from the target branch) alone produces the relevant warning message. That is, the logic has changed to this:

if mr_branch_diverged_from_target_branch:
  render "Update your branch with latest changes from target branch"
else if target_branch_security_report_is_older_than_7_days:
  render "Run a new pipeline for target branch"

This means that the mr_branch_diverged_from_target_branch warning will now almost always be shown for highly active projects. This is because the security report for an MR is based on the comparison of the security report of the latest HEAD of the target branch and the HEAD of the MR branch, and therefore won't give entirely correct results if the two have diverged.

The target_branch_security_report_is_older_than_7_days message will continue to be shown with the same frequency.

This change also aligns the implementation with the documented behaviour.

While this is now more correct, it may be a bit more noisy than the previous behaviour. There is future work planned to use the merge base for the comparison instead, which would improve this further.

Finally, this also:

• refactors the component tests to conform a bit more to our current standards;
• replaces some Bootstrap utility classes with GitLab UI classess.

Addresses part of #267504 (closed).

Changelog: changed EE: true

Screenshots (strongly suggested)

Before	After

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• [-] I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• [-] I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• [-] I have tested this MR in all supported browsers, or it's not needed.
• [-] I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.