Use ubuntu repository for markdownlint

Now that `ubuntu-latest` is Ubuntu 24.04, see actions/runner-images#9691

.github/actions/ci-checks/action.yml

@@ -36,14 +36,7 @@ runs:
       run: ./scripts/publish.sh --dry-run
       shell: bash
     - if: ${{ contains(fromJSON(inputs.checks), 'markdown') }}
-      # TODO: Keep only the last command once ubuntu-latest is 24.04.
-      run: |
-        set -x
-        curl -Os http://ftp.de.debian.org/debian/pool/main/r/ruby-mdl/markdownlint_0.13.0-4_all.deb
-        sudo apt install ruby-kramdown-parser-gfm ruby-mixlib-{cli,config,shellout}
-        sudo dpkg -i markdownlint_0.13.0-4_all.deb
-        rm markdownlint_0.13.0-4_all.deb
-        ./scripts/wrapper.sh mdl -g -s markdownlint.rb .
+      run: ./scripts/wrapper.sh mdl -g -s markdownlint.rb .
       shell: bash
     - if: ${{ contains(fromJSON(inputs.checks), 'taplo') }}
       run: ./scripts/ci-taplo.sh