Update release job CI Ruby version (#1799)

Author: nkzou

.github/workflows/prepare_release.yml

@@ -31,10 +31,10 @@ jobs:
           git config user.name "${GIT_AUTHOR_NAME}"
           git config user.email "${GIT_AUTHOR_EMAIL}"
 
-      - name: Set up Ruby 2.6
+      - name: Set up Ruby 2.7
         uses: ruby/setup-ruby@v1
         with:
-          ruby-version: 2.6
+          ruby-version: 2.7
 
       - name: Calculate version
         id: get_version