feat!: reusable workflow on python 3.9 (#430)

### Description
This release of reusable workflow switches build and development Python
version to Python 3.9.
It means that TAs not compatible with Python 3.9 will not work and tests
will be not executed on Splunk without Python 3.9 support.

Changes:

- Update entire workflow with downstreams to Python 3.9
- Upgrade Poetry to v2.1.4
- Splunk matrix feature filtering
- minor fixes

Note:

- Due to Poetry upgrade, TAs `pyproject.toml` needs to add
`package-mode` entry
https://python-poetry.org/docs/pyproject/#package-mode
- Reusable workflow v5 executes tests ONLY on Splunk with default Python
3.9 (9.3+). In order to run tests on older Splunk TA has to use reusable
workflow v4

### Checklist

- [ ] `README.md` has been updated or is not required
- [ ] push trigger tests
- [ ] manual release test
- [ ] automated releases test
- [x] pull request trigger tests
- [ ] schedule trigger tests
- [ ] workflow errors/warnings reviewed and addressed

### Testing done 

https://github.com/splunk/splunk-add-on-for-microsoft-office-365/actions/runs/16805896727

https://github.com/splunk/splunk-add-on-for-google-workspace/actions/runs/16804673207

https://github.com/splunk/splunk-add-on-for-salesforce/actions/runs/16804821563
https://github.com/splunk/splunk-add-on-for-box/actions/runs/16804906756

BREAKING CHANGE: switch to Python 3.9 and Poetry upgrade

Author: mkolasinski-splunk