Fix ruby version not found. (#414)

* Fix ruby version for Danger not found.

* Update ruby for Danger to 2.7.8.

* Also update ruby for XCTests jobs to 2.7.8

Author: Zandor300

.github/workflows/main.yml

@@ -18,7 +18,7 @@ jobs:
       - uses: ruby/setup-ruby@v1
         with:
           bundler-cache: true
-          ruby-version: 2.6.3
+          ruby-version: 2.7.8
 
       - uses: MeilCli/danger-action@v5
         with:
@@ -56,7 +56,7 @@ jobs:
       - uses: ruby/setup-ruby@v1
         with:
           bundler-cache: true
-          ruby-version: 2.6.3
+          ruby-version: 2.7.8
 
       - name: xcodebuild
         run: set -o pipefail && xcodebuild -scheme DeviceKit -destination "${{ matrix.platform }}" -configuration Debug ONLY_ACTIVE_ARCH=YES -enableCodeCoverage YES test