Update GH workflows to use latest ubuntu

Author: jonludlam

.github/workflows/changelog.yml

@@ -8,6 +8,6 @@ on:
 jobs:
   Changelog-Entry-Check:
     name: Check Changelog Action
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     steps:
       - uses: tarides/changelog-check-action@v1

.github/workflows/pr-number.yml

@@ -5,6 +5,6 @@ on: [pull_request_target]
 jobs:
   PR-Number-Update:
     name: Update PR number
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     steps:
       - uses: tarides/[email protected]