(PA-5336) Update task acceptance Ruby to 3.1

This commit updates the version of Ruby used by the task acceptance
tests from 2.5, which is end-of-life and no longer used by any supported
version of Puppet, to 3.1, which is the minimum version for Puppet 8.

Author: mhashizume

.github/workflows/task_acceptance_tests.yaml

@@ -15,7 +15,7 @@ jobs:
         os: [ 'centos-7', 'ubuntu-20.04', 'rocky-8' ]
 
     env:
-      ruby_version: 2.5
+      ruby_version: '3.1'
       GEM_BOLT: true
       BEAKER_debug: true
       BEAKER_set: docker/${{ matrix.os }}