[shiftstack] Clone shiftstack-qa from GitHub instead of GerritHub

[imatza-rh]

Summary

shiftstack-qa moved code review to GitHub PRs (shiftstack/shiftstack-qa#2). The ci-framework shiftstack role default still clones from GerritHub, which is no longer maintained.

• Change cifmw_shiftstack_qa_repo default to https://github.com/shiftstack/shiftstack-qa
• Rename cifmw_shiftstack_qa_gerrithub_change to cifmw_shiftstack_qa_change_ref - supports both GitHub PR refs (refs/pull/N/head) and GerritHub change refs (refs/changes/...)

Test plan

• Verified git clone + git fetch refs/pull/N/head works end-to-end with GitHub URL
• Confirmed zero external references to old variable name
• ansible-lint passed
• Testproject validation with Depends-On pointing to this PR

[openshift-ci]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[openshift-ci]

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