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]

Adding the "do-not-merge/release-note-label-needed" label because no release-note block was detected, please follow our release note process to remove it.

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