iserver-test: Add 9.2 test and some improvements

[ericywl]

Motivation/summary

• Add 9.2 upgrade paths manually, since we don't have integrationservertest: Automate upgrade paths #17602 yet
• Add input option in workflow dispatch to control which tests to run
• Add retry for telemetrygen in case of connection issues during ingestion
• Remove flush trigger since it seems to interfere with metrics aggregation sometimes

How to test these changes

Run workflow: https://github.com/elastic/apm-server/actions/runs/16346566007

[github-actions]

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[mergify]

This pull request is now in conflicts. Could you fix it @ericywl? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b iservertest-add-9.2-test upstream/iservertest-add-9.2-test
git merge upstream/main
git push upstream iservertest-add-9.2-test

[v1v]

I recall some edge cases when using workflow_dispatch default inputs and the schedule. Can you double-check if the default values are honoured when the schedule event runs?

I did run a quick search and found this StackOverflow thread or this

[ericywl]

I see, will try another way to do it instead. Thanks for the heads up!