ci: use -slim runner for small jobs

No need to spin up full VM runners for small automation tasks, when we
can use single-CPU runners in containers instead.

https://docs.github.com/en/actions/reference/runners/github-hosted-runners#single-cpu-runners

(some of this will potentially not work, because dependencies in the
slim image might not be available - however, it makes no sense to create
this as a PR, because all jobs touched here run on branches only. Thus
pushing directly to main)

.github/workflows/backport.yaml

@@ -9,7 +9,7 @@ on:
 jobs:
   backport:
     name: Backport
-    runs-on: ubuntu-24.04
+    runs-on: ubuntu-slim
     # It triggers only when PR is already merged on either:
     #
     # - The merge event itself (action != labeled) or

.github/workflows/ci.yaml

@@ -43,7 +43,7 @@ jobs:
       group: ci-tag-${{ (github.ref == 'refs/heads/main' && github.ref) || github.run_id }}
       cancel-in-progress: true
     if: vars.RELEASE_ENABLED
-    runs-on: ubuntu-24.04
+    runs-on: ubuntu-slim
     needs:
       - docs
       - test

.github/workflows/release.yaml

@@ -21,7 +21,7 @@ jobs:
 
   prepare:
     name: Prepare
-    runs-on: ubuntu-24.04
+    runs-on: ubuntu-slim
     needs:
       - build
     steps:
@@ -59,7 +59,7 @@ jobs:
     name: GitHub
     permissions:
       contents: write
-    runs-on: ubuntu-24.04
+    runs-on: ubuntu-slim
     needs:
       - prepare
     steps:
@@ -129,7 +129,7 @@ jobs:
 
   docker:
     name: Docker Hub
-    runs-on: ubuntu-24.04-arm
+    runs-on: ubuntu-slim
     needs:
       - prepare
     if: |
@@ -180,7 +180,7 @@ jobs:
 
   docker-description:
     name: Docker Hub Description
-    runs-on: ubuntu-24.04
+    runs-on: ubuntu-slim
     if: |
       vars.DOCKER_REPO && vars.DOCKER_USER &&
       github.ref == 'refs/tags/devel'