ci: Remove the Ubuntu 18.04 runner

Github has announced[1] that it is going to remove support for the Ubuntu
18.04 runner. It will start introducing temporary failures at increasing
frequency in October 22 and completely remove support for it in April 23.

To avoid CI failing remove the Ubuntu 18.04 runner and add the 22.04 runner
instead.

[1] https://github.blog/changelog/2022-08-09-github-actions-the-ubuntu-18-04-actions-runner-image-is-being-deprecated-and-will-be-removed-by-12-1-22/

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>

.github/workflows/test.yml

@@ -44,8 +44,8 @@ jobs:
       fail-fast: false
       matrix:
         os: [
-          '18.04',
-          '20.04'
+          '20.04',
+          '22.04'
         ]
     runs-on: ubuntu-${{ matrix.os }}
     name: '🐧 Ubuntu ${{ matrix.os }}'