Replace obsolete 'Generate Token' step

Working on #44 -
now that scala-steward-org/scala-steward-action#428
updated the Scala Steward GitHub Action to have its _own_ support for
GitHub App authentication, we can remove the additional 'Generate token'
step from our GitHub Action workflow, with these benefits:

* No longer need to use the tibdex/github-app-token - one fewer action
  we need to trust
* In a subsequent step, we may be able to remove the necessity of
  adding repos to `REPOSITORIES.md` - as, so far as I can see, the GitHub
  Action will now automatically run on all repos we install it on, without
  having to also add them to `REPOSITORIES.md`:
  https://github.com/scala-steward-org/scala-steward/blob/113884ac11d639ed8b70fc368f6a69895b14f8ba/modules/core/src/main/scala/org/scalasteward/core/application/StewardAlg.scala#L97-L98

.github/workflows/public-repos-scala-steward.yml

@@ -11,5 +11,6 @@ jobs:
     uses: ./.github/workflows/reusable-scala-steward.yml
     with:
       app_id: 214238
+      app_installation_id: 26822732
     secrets:
       private_key: ${{ secrets.SCALA_STEWARD_APP_PRIVATE_KEY }}

.github/workflows/reusable-scala-steward.yml

@@ -6,6 +6,9 @@ on:
       app_id:
         required: true
         type: string
+      app_installation_id:
+        required: true
+        type: string
     secrets:
       private_key:
         required: true
@@ -22,19 +25,13 @@ jobs:
         with:
           repository: guardian/scala-steward-public-repos
           path: common-config
-      - name: Generate token
-        id: generate-token
-        uses: tibdex/github-app-token@v1
-        with:
-          app_id: ${{ inputs.app_id }}
-          private_key: ${{ secrets.private_key }}
 
       - name: Execute Scala Steward
         uses: scala-steward-org/[email protected]
 
         with:
-          # Use the token generated by the previous step and the 
-          # Guardian Scala Steward GitHub App
-          github-token: ${{ steps.generate-token.outputs.token }}
-          repos-file: REPOSITORIES.md
+          github-app-id: ${{ inputs.app_id }}
+          github-app-installation-id: ${{ inputs.app_installation_id }}
+          github-app-key: ${{ secrets.private_key }}
+          repos-file: REPOSITORIES.md # possibly no longer necessary, thanks to `github-app-*` configuration
           repo-config: common-config/scala-steward.conf # from checkout of guardian/scala-steward-public-repos