Update CI target OSes

Remove platforms deprecated by GitHub Actions
Add new platforms supported by the same

.github/workflows/update-master-docs.yml

@@ -17,7 +17,7 @@ on:
 jobs:
   build:
     if: github.repository_owner == 'gbdev'
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-22.04
     steps:
       - name: Checkout rgbds@master
         uses: actions/checkout@v2
@@ -31,16 +31,10 @@ jobs:
           repository: gbdev/rgbds-www
           ref: master
           path: rgbds-www
-      - name: Build and install mandoc + install groff
+      - name: Install groff and mandoc
         run: |
           sudo apt-get -qq update
-          sudo apt-get install -yq groff zlib1g-dev
-          wget 'http://mandoc.bsd.lv/snapshots/mandoc-1.14.6.tar.gz'
-          tar xf mandoc-1.14.6.tar.gz
-          cd mandoc-1.14.6
-          ./configure
-          make
-          sudo make install
+          sudo apt-get install -yq groff mandoc
       - name: Update pages
         working-directory: rgbds/man
         run: |