(maint) Set acceptance workflow Rubygems version

On running the acceptance workflow, we get a warning about a bug
in the currently running version of Rubygems (3.0.3.1) that affects
other code in the workflow.

This commit sets the Rubygems version in the acceptance workflow to
3.3.26, the same version of Rubygems that we use in Puppet GitHub
Actions.

Author: mhashizume

.github/workflows/acceptance_tests.yml

@@ -32,6 +32,7 @@ jobs:
         uses: ruby/setup-ruby@v1
         with:
           ruby-version: '2.7'
+          rubygems: '3.3.26'
 
       - name: Fix common Linux and macOS permissions
         if: runner.os != 'Windows'