Parallelize GitHub Action tests via a customTest task

[sovdeeth]

Problem

Now that we're supporting 18 months of versions on Java 21, the test times are getting overly long since it's all a single action (7+ minutes).

Solution

This pulls from experiments done in #8254 and adds a test task (customTest) that runs tests based on provided properties rather than hardcoded values. This allows the github action parallelize-tests to create a matrix of test environments split over N actions, which then runs a subset of the total environments. Given each test takes about 60s, plus the initial build that takes 30s, this cuts runtime for 8 versions from 8:30 to 3:30 if split 3 ways, or 2:30 if split 4. I've chosen split 3 ways to not clog up the check tab too much.

Also added 1.21.1 test env for full coverage

Testing Completed

Manual confirmation of the customTest task working, plus the check tab of this pr

Supporting Information

---

Completes: none
Related: none
AI assistance: none