Migrate Bootstrap dropdown to GitLab UI GlDropdown in comment_form.vue

What does this MR do?

• Replaces the deprecated dropdown used in 'comment_form.vue' with GitLab UI GlDropdown.

  'comment_form.vue' is used in issue, epic and merge request.

• Updates the selectors used in the specs: 'comment_form_spec.js' and 'discussion_comments_shared_example.rb'.

  The shared examples under thread comments are split into two:

  thread comments for commit and snippet - this is exactly thread comments minus the parts that were only relevant to issue, epic and merge requests; e.g., !50933 (diffs).

  thread comments for issue, epic and merge request - identical to the original thread comments but has updated selectors

Completes #235366

Screenshots (strongly suggested)

location	Before (Bootstrap)	After (GitLab UI)
issue
*
*
epic
merge request

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• 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