Move time-sensitive Actions workflows to ubuntu-latest

We originally moved these to `ubuntu-slim`, but there is a significant performance difference.  Since we often find ourselves waiting on these jobs, let's use the faster runners.

.github/workflows/post-release-mergeback.yml

@@ -24,7 +24,7 @@ defaults:
 
 jobs:
   merge-back:
-    runs-on: ubuntu-slim
+    runs-on: ubuntu-latest
     environment: Automation
     if: github.repository == 'github/codeql-action'
     env:

.github/workflows/prepare-release.yml

@@ -29,7 +29,7 @@ defaults:
 jobs:
   prepare:
     name: "Prepare release"
-    runs-on: ubuntu-slim
+    runs-on: ubuntu-latest
     if: github.repository == 'github/codeql-action'
 
     permissions:

.github/workflows/update-bundle.yml

@@ -20,7 +20,7 @@ defaults:
 jobs:
   update-bundle:
     if: github.event.release.prerelease && startsWith(github.event.release.tag_name, 'codeql-bundle-')
-    runs-on: ubuntu-slim
+    runs-on: ubuntu-latest
     permissions:
       contents: write # needed to push commits
       pull-requests: write # needed to create pull requests

.github/workflows/update-release-branch.yml

@@ -26,7 +26,7 @@ jobs:
 
   update:
     timeout-minutes: 45
-    runs-on: ubuntu-slim
+    runs-on: ubuntu-latest
     if: github.event_name == 'workflow_dispatch'
     needs: [prepare]
     env:
@@ -77,7 +77,7 @@ jobs:
 
   backport:
     timeout-minutes: 45
-    runs-on: ubuntu-slim
+    runs-on: ubuntu-latest
     environment: Automation
     needs: [prepare]
     if: ${{ (github.event_name == 'push') && needs.prepare.outputs.backport_target_branches != '[]' }}