Add ruby-version to publish workflow

The ruby/setup-ruby action needs a version of Ruby specified to work.
Previously it was reading the `.ruby-version` file, however we removed
that in commit 2aa58cc. We could restore that file, but because
`ruby-install` doesn't support loose version specifiers, we'd have to
keep it maintained. Instead we just tell the action to install the
latest version of Ruby 3.

Author: lfdebrux

.github/workflows/publish.yaml

@@ -19,6 +19,8 @@ jobs:
       - uses: actions/checkout@v3
 
       - uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: '3'
 
       - name: Check if new version to release
         id: gem_version
@@ -53,6 +55,7 @@ jobs:
 
       - uses: ruby/setup-ruby@v1
         with:
+          ruby-version: '3'
           bundler-cache: true
 
       - name: Publish