Backport of Mw/add backport checker into release/1.0.x

[hc-github-team-consul-core]

Backport

This PR is auto-generated from #1982 to be assessed for backporting due to the inclusion of the label backport/1.0.x.

The below text is copied from the body of the original PR.

---

Changes proposed in this PR:

• Add a pipeline check for backport labels. This will ensure that developers must either add a backport label or deliberately specify that this pr does not require backporting with the pr/no-backport label.
• This check simply checks for the presence of a label matching backport/*. It is up to the developer to add the right backport labels as appropriate for a given PR.

How I've tested this PR:

No Backport label and no pr/no-backport

With backport label

With pr/no-backport label

How I expect reviewers to test this PR:

Checklist:

• Tests added
• CHANGELOG entry added

  HashiCorp engineers only, community PRs should not add a changelog entry.
  Entries should use present tense (e.g. Add support for...)

---

Overview of commits

• c740fed - d482dd9