Update JRuby to 9.4.8.0

Puppetserver recently updated the JRuby version used in the 8.x stream
from JRuby 9.4.7.0 to 9.4.8.0: puppetlabs/jruby-utils@0e23173

This commit updates Facter's tests to JRuby 9.4.8.0 to match.

Author: mhashizume

.github/workflows/unit_tests.yaml

@@ -23,7 +23,7 @@ jobs:
           - '3.0'
           - '3.2'
           - 'jruby-9.3.15.0'
-          - 'jruby-9.4.7.0'
+          - 'jruby-9.4.8.0'
     runs-on: ubuntu-20.04
     steps:
       - name: Checkout current PR