chore: Updated Python image to 3.8 in backend/Dockerfile

[hbelmiro]

Description of your changes:
This PR updates backend/Dockerfile to use Python 3.8 since Python 3.7 is no longer supported by https://bootstrap.pypa.io/pip/3.7/get-pip.py.

ERROR: This script does not work on Python 3.7 The minimum supported Python version is 3.8. Please use https://bootstrap.pypa.io/pip/3.7/get-pip.py instead.
Error: building at STEP "RUN wget https://bootstrap.pypa.io/get-pip.py && python3 get-pip.py": while running runtime: exit status 1

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[google-oss-prow]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[hbelmiro]

cc @chensun

[google-oss-prow]

@hbelmiro: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
kubeflow-pipeline-upgrade-test	c4891d1	link	false	/test kubeflow-pipeline-upgrade-test

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[chensun]

/lgtm

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: chensun

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• backend/OWNERS [chensun]

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[gmfrasca]

/lgtm