Set ArtifactV4Upgrade's default value to true

[angelapwen]

artifacts@v3 has an official deprecation date of December 5. Technically, merging this PR does not actually change much — the feature flags will first check the GitHub API for the value of the feature flag (which has been rolled out at 100% enabled for several weeks).

In the future, we will remove the actual feature flag in GitHub's internal feature flag portal. Once this is done, the default true value set in this PR will be actually used in production. We don't want to yet remove the feature flag from the internal feature flag portal, as old versions of the CodeQL Action will use a default value of false if the flag is not found from the GItHub API, and use the deprecated version of actions@v3. Therefore we will only be able to remove the feature flag from the internal feature flag portal once we no longer support these older versions of the Action.

Note that GHES customers will continue to use artifacts@v3 until it is supported on GHES.

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.

[aeisenberg]

LGTM!