(MODULES-11365) Add puppet8 to nightly gem tests

This commit removes Puppet 5 and adds Puppet 8 to the unit tests
with nightly Puppet gems.

Author: mhashizume

.github/workflows/unit_tests_with_nightly_puppet_gem.yaml

@@ -13,12 +13,12 @@ jobs:
     strategy:
       matrix:
         os: [ 'ubuntu-20.04', 'macos-latest', 'windows-2022' ]
-        puppet_version: [ 6, 7 ]
+        puppet_version: [ '7', '8' ]
         include:
-          - puppet_version: 6
-            ruby: 2.5
-          - puppet_version: 7
-            ruby: 2.7
+          - puppet_version: '7'
+            ruby: '2.7'
+          - puppet_version: '8'
+            ruby: '3.2'
 
           - os: 'ubuntu-20.04'
             os_type: 'Linux'