workflows/labels: manage "first-time contribution" label

This is currently managed with yet another bot, but we don't need to -
this fits into our workflow nicely.

(cherry picked from commit 28f3b92403)

.github/workflows/labels.yml

@@ -234,6 +234,8 @@ jobs:
                       '12.approvals: 2': approvals.size == 2,
                       '12.approvals: 3+': approvals.size >= 3,
                       '12.approved-by: package-maintainer': Array.from(maintainers).some(m => approvals.has(m)),
+                      '12.first-time contribution':
+                        [ 'NONE', 'FIRST_TIMER', 'FIRST_TIME_CONTRIBUTOR' ].includes(pull_request.author_association),
                     }
                   )