chore(ci): Add auto-approve step for PRs by renovate[bot] (#2839)

* chore(ci): Add auto-approve step for PRs by renovate[bot]

# Issue

After activating the CFF default `main` branch protection rule, reviews
get dismissed if the PR diff changes. As our automated dependency PR
post-processing might do exactly that, it needs to reapprove the PR.

# Fix

Auto-approve dependabot PRs if our PR post-processing has modified the
PR.

# Remarks

Addtionally, this PR removes obsolete workflows:

- `.dependency-spec-updates.yaml` became obsolete with the introduction of `dependency-update.go-mod-tidy_and_make-package-specs.yaml` and never worked afterwards: https://github.com/cloudfoundry/app-autoscaler-release/actions/workflows/dependency-spec-updates.yaml - the green runs are where it didn't have to do anything because it was already taken care of by `dependency-update.go-mod-tidy_and_make-package-specs.yaml`
- `dependency-updates-auto-merge.yaml` had two jobs: Approving `dependabot` PRs, which we disabled in favor of `renovate` and enabling automerge which `renovate` also takes care of itself.

.github/workflows/dependency-update.go-mod-tidy_and_make-package-specs.yaml → .github/workflows/dependency-updates-post-processing.yaml

@@ -1,4 +1,4 @@
-name: "renovate-/ospo-bot: go mod tidy and make package-specs"
+name: "Dependency Updates Post-Processing"
 on:
   pull_request:
     branches:
@@ -22,8 +22,9 @@ on:
       - "src/**/go.sum"
   workflow_dispatch: {}
 jobs:
-  pkgs-specs_and_go-mod-tidy:
-    name: "go mod tidy && make package-specs"
+  dependency-updates-post-processing:
+    if: github.actor == 'renovate[bot]'
+    name: "Dependency Updates Post-Processing"
     runs-on: ubuntu-latest
     container:
       image: ghcr.io/cloudfoundry/app-autoscaler-release-tools:main
@@ -103,3 +104,5 @@ jobs:
             echo 'No files changed!'
           fi
           echo '🏁'
+      - name: Auto-approve
+        uses: hmarr/auto-approve-action@v4