Revert pull request labeler version upgrade (#2242)

#2188 broke it, and I
don't feel like fixing it right now

I case someone feel like fixing it, the documentation is here:
https://github.com/actions/labeler/tree/main

Bear in mind that renovate bot is going to try to recreate the PR to
upgrade this action version again. We'll just have to tell it to skip
this one.

.github/workflows/labeler.yml

@@ -20,4 +20,4 @@ jobs:
       pull-requests: write
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/labeler@v5
+    - uses: actions/labeler@v4