Only run Py39 and Py313 tests for PostCommit Arm

[Abacn]

This workflow is running on github-hosted GitHub Action runner (assuming resource is more limited), and historically being very flaky. Currently flaky due to random tests failing.

Decrease the matrix to run only two Python versions. We have https://github.com/apache/beam/actions/workflows/beam_Python_ValidatesContainer_Dataflow_ARM.yml?query=event%3Aschedule exercises the arm container for each Python version already, no need to run full PostCommit suite for every Python version on ARM

Testing in https://github.com/apache/beam/actions/runs/18502517306

mitigate #30760

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[gemini-code-assist]

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[Abacn]

R: @Amar3tto

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[Abacn]

R: @liferoad