Fix: Switch to Ubuntu 22.04 for CI workflows

ci.yml currently requests Ubuntu 18.04 runners, but per actions/runner-images#6002 Ubuntu 18.04 runners are now fully unsupported:
```
We have started the deprecation process for Ubuntu 18.04.
[...] the image will be fully unsupported by 2023/04/03
```

.github/workflows/ci.yml

@@ -4,7 +4,7 @@ on: [push, pull_request]
 
 jobs:
   testsuite:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-22.04
     strategy:
       fail-fast: false
       matrix:
@@ -42,4 +42,4 @@ jobs:
       if: matrix.php-version == '7.4'
       run: |
         wget https://scrutinizer-ci.com/ocular.phar
-        php ocular.phar code-coverage:upload --format=php-clover coverage.clover;
+        php ocular.phar code-coverage:upload --format=php-clover coverage.clover;