Disable functional tests in CI for now

We rely on VirtualBox running in a GitHub Actions macOS runner image for
our functional (Cucumber) tests.

There was a long-standing issue in the GitHub Actions runner image that
caused VirtualBox to fail, breaking our CI builds. This issue has
recently been fixed with a new version of the macOS runner image.

actions/runner-images#8730

However, another GitHub Actions bug means that our CI builds are not
getting the newest macOS runner image. So our builds continue to fail.

actions/runner-images#8642

Until this is resolved, I am disabling functional tests in CI so that
contributors aren't blocked by failing builds that they can't do
anything to fix.

.github/workflows/ci.yml

@@ -57,57 +57,3 @@ jobs:
           bundler-cache: true
       - name: Run rubocop
         run: bundle exec rake lint
-
-  functional:
-    runs-on: macos-12
-    strategy:
-      matrix:
-        ruby:
-          [
-            "2.0",
-            "2.1",
-            "2.2",
-            "2.3",
-            "2.4",
-            "2.5",
-            "2.6",
-            "2.7",
-            "3.0",
-            "3.1",
-            "3.2",
-            "head",
-          ]
-    steps:
-      - uses: actions/checkout@v4
-
-      - name: Cache Vagrant boxes
-        uses: actions/cache@v3
-        with:
-          path: ~/.vagrant.d/boxes
-          key: ${{ runner.os }}-vagrant-v2-${{ hashFiles('Vagrantfile') }}
-          restore-keys: |
-            ${{ runner.os }}-vagrant-v2-
-
-      - name: Run vagrant up
-        run: vagrant up
-
-      - name: Set up Ruby
-        uses: ruby/setup-ruby@v1
-        with:
-          ruby-version: ${{ matrix.ruby }}
-          bundler-cache: true
-
-      - name: Run functional tests
-        run: bundle exec rake test:functional
-
-  functional-all:
-    runs-on: ubuntu-latest
-    needs: [functional]
-    if: always()
-    steps:
-      - name: All tests ok
-        if: ${{ !(contains(needs.*.result, 'failure')) }}
-        run: exit 0
-      - name: Some tests failed
-        if: ${{ contains(needs.*.result, 'failure') }}
-        run: exit 1