Remove update-proxy-release workflow

[mbg]

This workflow is no longer needed now that the update-job-proxy binaries are published as part of CLI releases.

Risk assessment

For internal use only. Please select the risk level of this change:

• Low risk: Changes are fully under feature flags, or have been fully tested and validated in pre-production environments and are highly observable, or are documentation or test only.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Consider adding a changelog entry for this change.
• Confirm the readme and docs have been updated if necessary.

[Copilot]

Pull Request Overview

This PR removes the update-proxy-release workflow that was previously used to update proxy binaries, as this functionality is now handled as part of CLI releases.

• Removes the entire update-proxy-release.yml workflow file
• Eliminates the need for manual workflow dispatch to update proxy binaries
• Streamlines the release process by consolidating proxy binary publishing