derive ruby version from .ruby-version file

Author: johnjeffers

.github/workflows/deploy.yaml

@@ -50,10 +50,9 @@ jobs:
           21=${JAVA_HOME}
           EOF
 
+      # Installs the version specified in the .ruby-version file in the repo root.
       - name: install ruby
         uses: ruby/setup-ruby@v1
-        with:
-          ruby-version: 2.7.5
 
       - name: compile
         shell: bash -l {0}