Replace Bootstrap popover with GitLab UI popover for merge conflict

What does this MR do?

• Updated to use GlPopover instead of jQuery bootstrap popover.
• Fixes href boolean console warning.
• Replaces classes use with data-testid to better communicate use.
• Centers the link text.
• Resolves issue where it looks like the background became grey since implementation
• Polishes test spec.

To reproduce locally(gdk)

1. Create a merge conflict(if you use the seeded gdk data - the Cannot be automatically merged MR in gitlab-org/gitlab-test is a good candidate)
2. Go to Settings -> Repository on the project and Expand Protected branches and Protect the Source branch that the MR is using.
3. Go back to the merge request and refresh, then hover over the now disabled Resolve conflicts button.

Screenshots (strongly suggested)

Note

I had to enable the button for Resolve conflicts in the code to even get the before to show the popup. (I'm not entirely certain if it is broken/buggy or otherwise - so that is why the button looks enabled in the view below - either way, it is fixed now)

before	after

Console error addressed with change to href

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #227257