Modernize README badge and install instructions

[samarthDevops]

Title: Modernize README badge and install instructions

Summary

• point the status badge at the GitHub Actions test workflow
• recommend python -m pip install --upgrade . when installing from source

Testing

• not applicable (documentation-only change)

Does this PR introduce a user-facing change?

NONE

[k8s-ci-robot]

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[yliaog]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: samarthDevops, yliaog

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[yliaog]

/release-note-none