fix: Update CI to use Ubuntu 22.04 as the minimum

Ubuntu 20.04 is no longer supported by GitHub Actions

Author: varungandhi-src

.github/actionlint.yaml

@@ -1,4 +1,4 @@
 # Stop actionlint from triggering failures
 self-hosted-runner:
   labels:
-    - ubuntu-20.04-16core-graph-team
+    - ubuntu-22.04-32core-graph-team-amd64

.github/workflows/build_gems.yml

@@ -6,7 +6,7 @@ jobs:
     name: 'Build gem'
     strategy:
       matrix:
-        platform: ['ubuntu-20.04', 'macos-13']
+        platform: ['ubuntu-22.04', 'macos-13']
         config: ['asserts', 'release']
     runs-on: ${{ matrix.platform }}
     env: