Quote ruby versions in yaml

Without quotes these are parsed as numbers, which can cause subtle
problems. For example, "ruby: 3.0" is interpreted by the
ruby/setup-rubyaction as "ruby: 3", meaning 3.x instead of the
intended 3.0.x

.github/workflows/lint.yml

@@ -7,7 +7,7 @@ concurrency:
   cancel-in-progress: true
 env:
   os: ubuntu-20.04
-  ruby: 2.7
+  ruby: '2.7'
 jobs:
   rubocop:
     name: RuboCop

.github/workflows/tests.yml

@@ -11,7 +11,7 @@ jobs:
     strategy:
       matrix:
         ubuntu: [20.04, 22.04]
-        ruby: [2.7, 3.0, 3.1]
+        ruby: ['2.7', '3.0', '3.1']
     runs-on: ubuntu-${{ matrix.ubuntu }}
     env:
       RAILS_ENV: test