.github/workflows: Bump us up to Ubuntu 22.04, as 18.04 is deprecated

.github/workflows/pr-checks.yml

@@ -13,7 +13,7 @@ on:
 
 jobs:
   flake8:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-22.04
 
     steps:
 

.github/workflows/push-checks.yml

@@ -14,7 +14,7 @@ on:
 jobs:
   build:
 
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-22.04
 
     steps:
     - uses: actions/checkout@v3