ci: refresh GitHub Actions matrix

 * Add ubuntu-22.04, remove 18.04
 * Add macos-12, remove 10.15

Change-Id: I66908bb493100d9c7a6edda2a72e3a536b81b337
diff --git a/.github/workflows/docs.yml b/.github/workflows/docs.yml
index 37b8c1c..5e532d4 100644
--- a/.github/workflows/docs.yml
+++ b/.github/workflows/docs.yml
@@ -11,11 +11,11 @@
 
 jobs:
   build:
+    runs-on: ${{ matrix.os }}
     strategy:
       fail-fast: false
       matrix:
-        os: [macos-11, ubuntu-20.04]
-    runs-on: ${{ matrix.os }}
+        os: [macos-12, ubuntu-20.04]
     env:
       JOB_NAME: Docs
     steps: