feat: parametrized python version (#426)

### Description

Add `dev-python-version` input parameter with default value 3.9.
This pull request updates the
`.github/workflows/reusable-build-test-release.yml` workflow to make the
Python version used for development and testing configurable, rather
than hardcoded. It removes redundant jobs, standardizes Python version
usage across all steps, and ensures downstream actions and test runners
receive the correct Python version. These changes improve
maintainability and flexibility for future Python version upgrades.

**Python version configuration and usage:**
* Added a new input parameter `dev-python-version` (default: `"3.9"`) to
allow dynamic selection of the Python version for all jobs and steps,
replacing hardcoded values.
* Updated all `setup-python` and related steps to use `${{
inputs.dev-python-version }}` instead of fixed versions, including for
poetry installation and requirements export.

**Workflow simplification:**

* Removed redundant jobs (`run-unit-tests-3_9`, `build-3_9`) in favor of
a single, parameterized job structure that uses the dynamic Python
version.

**Downstream compatibility:**

* Updated all test runner steps (`splunk/wfe-test-runner-action`) to
pass the selected Python version via the new input and environment
variable, and switched to a version of the runner that supports dynamic
Python version selection.

**Dependency management:**

* Updated installation of `splunk-packaging-toolkit` to specify a fixed
version (`1.2.4`) for greater reliability.

### Checklist

- [x] `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 
python 3.7

https://github.com/splunk/splunk-add-on-for-microsoft-office-365/actions/runs/16486208211
python 3.9

https://github.com/splunk/splunk-add-on-for-salesforce/actions/runs/16471291450

https://github.com/splunk/splunk-add-on-for-microsoft-office-365/actions/runs/16471664299

https://github.com/splunk/splunk-add-on-for-google-workspace/actions/runs/16745786889

splunk/splunk-add-on-for-google-cloud-platform#849

splunk/splunk-add-on-for-microsoft-cloud-services#1322
python 3.13

https://github.com/splunk/splunk-add-on-for-microsoft-office-365/actions/runs/16443769270

### Related changes
https://github.com/splunk/wfe-test-runner-action/pull/37/files
https://github.com/splunk/ta-automation-app-of-apps/pull/29/files
https://cd.splunkdev.com/workflow-engine/scs/images/-/merge_requests/180

Author: mkolasinski-splunk