Proposal: Merge jenkins jobs test.getDependency and systemtest.getDependency

**Summary**

This issue is to propose and discuss fusing jenkins jobs [test.getDependency](https://ci.adoptium.net/job/test.getDependency/) and [systemtest.getDependency](https://ci.adoptium.net/job/systemtest.getDependency/)

**Subtasks**

- [ ] Add systemtest dependencies into the test.getDependency array and remove systemtest-specific logic from getDependencies.pl in TKG repo.
- [ ] Run the new test.getDependency job to populate the artifacts list.
- [ ] Remove all references to systemtest.getDependency
- [ ] Rename test.getDependency to test.getDependencies (as we are "getting" more than one dependency jar)
- [ ] Remove all references to the [systemtest_prereqs](https://github.com/search?q=org%3Aadoptium+%09systemtest_prereqs&type=code) folder (both for storage and retrieval)
- [ ] Consider if we should delete the systemtest.getDependency job (as doing so may break backwards compatibility).

**Notes**

Originally discussed here: https://github.com/adoptium/aqa-tests/pull/6753#issuecomment-3586864650

Start by comparing the Jenkins jobs test.getDependency and systemtest.getDependency, then inspect getDependencies.pl in the TKG repository and references to systemtest_prereqs. Done means the dependency handling is unified, references and storage paths are updated, the renamed job populates the artifacts list, and backwards-compatibility implications are resolved.

Written by the indexing model from the issue text.