Update gem_release_prep.yml ruby. version from 2.7 to 3.1

Prior to this commit the workflow used ruby version 2.7. As this is old and our tools are now running on Ruby 3 I have increased the version to Ruby 3.1.

Author: pmcmaw

.github/workflows/gem_release_prep.yml

@@ -34,7 +34,7 @@ jobs:
       - name: "setup ruby"
         uses: "ruby/setup-ruby@v1"
         with:
-          ruby-version: "2.7"
+          ruby-version: "3.1"
           bundler-cache: "true"
 
       - name: "bundle environment"