ci: Add latest Penlight to testing matrix, drop unnecessary versions

This now only includes the newest, the oldest supported, and interim
releases with significant (non whitespace) changes to the pl.class()
system which is the main thing relevant to this project.

.github/workflows/busted.yml

@@ -9,7 +9,7 @@ jobs:
       fail-fast: false
       matrix:
         luaVersion: [ "5.4", "5.3", "5.2", "5.1", "luajit", "luajit-openresty" ]
-        penlightVersion: [ "1.9.2", "1.8.0", "1.6.0", "1.5.4" ]
+        penlightVersion: [ "1.12.0", "1.9.2", "1.8.0", "1.6.0", "1.5.4" ]
     runs-on: ubuntu-20.04
     steps:
     - name: Checkout