Increase parallel test jobs from 4 to 6

Improves CI speedup from 1.84x to 2.35x based on local testing.
Each runner now handles ~17 specs instead of ~26.

Author: mroderick

.github/workflows/ruby.yml

@@ -9,8 +9,8 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ci_node_total: [4]
-        ci_node_index: [0, 1, 2, 3]
+        ci_node_total: [6]
+        ci_node_index: [0, 1, 2, 3, 4, 5]
 
     env:
       # prevent unnecessary log output -- https://bundler.io/man/bundle-config.1.html