Fix versions of doxygen used on github actions jobs

The publish job uses `ubuntu-latest` which is currently Ubuntu 22.04.
Switching to specifying 22.04 allows us to manage the point as which the
versions are updated.

Updating the version of Ubuntu used for the doxygen-check job from
Ubuntu 20.04 to 22.04 matches the check job against the publishing job.

Author: thomasspriggs

.github/workflows/doxygen-check.yaml

@@ -6,7 +6,9 @@ on:
 jobs:
   # This job takes approximately 2 minutes
   check-doxygen:
-    runs-on: ubuntu-20.04
+    # Note that the versions used for this `check-doxygen` job should be kept in
+    # sync with the `publish` job.
+    runs-on: ubuntu-22.04
     steps:
     - uses: actions/checkout@v3
     - name: Fetch dependencies

.github/workflows/publish.yaml

@@ -4,7 +4,9 @@ on: [push, pull_request]
 jobs:
   # This job takes approximately 3 minutes
   publish:
-    runs-on: ubuntu-latest
+    # Note that the versions used for this `publish` job should be kept in sync
+    # with the `check-doxygen` job.
+    runs-on: ubuntu-22.04
     steps:
       - name: Checkout repository
         uses: actions/checkout@v3