Remove artifact_v4_upgrade feature flag logic and related PR checks

[angelapwen]

I was reminded that we can set the value in the API response to be true for old versions of the CodeQL Action; once that change is in, this PR can be merged.

The PR removes all mention of the feature flag, so the new artifact dependency will always be used except on GHES.

The CODEQL_ACTION_ARTIFACT_V4_UPGRADE environment variable is now deprecated as well so it is removed from the PR checks that included it. The debug-artifacts-legacy PR check, that makes sure that the deprecated legacy version of the client works, is now also removed as we no longer can set CODEQL_ACTION_ARTIFACT_V4_UPGRADE to false.

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.

[angelapwen]

I will merge this once the monolith PR change is deployed.