Remove remaining Python 3.8 Artifacts

[jrmccluskey]

Removes remaining artifacts around Python 3.8 support, calls out support deprecation in CHANGES.md

Closes #31192

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[jrmccluskey]

The changes to the Python 3.9 post-commit are the most substantial, running at https://github.com/apache/beam/actions/runs/11485756985/job/31966569465

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[jrmccluskey]

PostCommit failures on the branch are due to #32936

[damccorm]

Looks like this change broke the TensorRT tests - https://github.com/apache/beam/actions/workflows/beam_PostCommit_Python.yml

The reason is that TensorRT depends on a docker image -

beam/sdks/python/test-suites/dataflow/common.gradle

Line 410 in 24a0447

"sdk_container_image": "us.gcr.io/apache-beam-testing/python-postcommit-it/tensor_rt:latest",

- which is static, not built based on the test suite its in (this is actually what the comment in this file is saying).

I think we need to use

beam/sdks/python/test-suites/containers/tensorrt_runinference/tensor_rt.dockerfile

Line 25 in 24a0447

COPY --from=apache/beam_python3.10_sdk:latest /opt/apache/beam /opt/apache/beam

to build a new docker image for the tensorRT jobs and push it to us.gcr.io/apache-beam-testing/python-postcommit-it/tensor_rt:latest. @jrmccluskey would you mind following up and doing that? It looks like you already updated the dockerfile, so hopefully its smooth sailing