Use proxy artifacts for v2.21.1

[mbg]

This PR updates the start-proxy action to use update-job-proxy binaries that are attached as release artifacts to https://github.com/github/codeql-action/releases/tag/codeql-bundle-v2.21.1

I uploaded these manually from a recent build of the update-job-proxy that includes the new GOPROXY handler.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Confirm the readme has been updated if necessary.
• Confirm the changelog has been updated if necessary.

[Copilot]

Pull Request Overview

A PR that updates the start-proxy action to use a new set of update-job-proxy binaries from the codeql-bundle-v2.21.1 release.

• Updated the UPDATEJOB_PROXY_VERSION constant to the new version string.
• Updated the UPDATEJOB_PROXY_URL_PREFIX to point to the v2.21.1 release URL.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/start-proxy-action.ts	Updated version constant and URL prefix for proxy artifact download.
lib/start-proxy-action.js	Synchronized update of version constant and URL prefix with TS source.