Describe the undercoverage check in rspec:coverage for gitlab-org/gitlab

What does this MR do?

Updates the gitlab-org/gitlab troubleshooting entry for undercoverage failures in the gitlab-org-contribution skill.

gitlab-org/gitlab!265673 (merged) removed the rspec:undercoverage job. The rspec:coverage job now runs the check after it merges the coverage. The entry now:

• names the check in rspec:coverage, and notes that older pipelines still show rspec:undercoverage
• says that a failed check blocks tier 2 pipelines as well as tier 3, since gitlab-org/gitlab!265865 (merged), and otherwise exits with 202, which is allowed to fail
• says that pipeline:skip-undercoverage skips only the check, so the coverage report is still published
• cites gitlab-org/gitlab!265673 (merged) and gitlab-org/gitlab!265865 (merged), and the files at the commits that changed them (a04304a6 and e5ced89f)

Bumps the skill to 0.7.2.