Update Github Actions to ubuntu-latest

Co-authored-by: Martijn Laarman <[email protected]>

Author: github-actions[bot]

.github/workflows/backport.yml

@@ -7,7 +7,7 @@ on:
 
 jobs:
   backport:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
     name: Backport
     steps:
       - name: Backport