Enable analysis summary v2 and sub-language file coverage information on GHES

[henrymercer]

This PR:

• Enables analysis summary v2 on GHES 3.9 and later (since the new summary currently links to the status page unconditionally).
• Enables sub-language file coverage information on the upcoming GHES 3.12 and later.
• Cleans up the feature flag for exporting failed SARIF.

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]

Looks good, thanks for the 🧹 cleanup on failed SARIF too!

[henrymercer]

Now I get an opportunity to use #1971 :)

[henrymercer]

Well, I'm not too surprised that didn't work. Pushing a fix.

[angelapwen]

Ah.. I'd also thought that it'd check out the current branch by default, but it does make sense that the checkout action isn't aware of the current branch ref.

[henrymercer]

Ah.. I'd also thought that it'd check out the current branch by default, but it does make sense that the checkout action isn't aware of the current branch ref.

I think it checks out the synthetic merge commit vs the head of the branch, and the latter is what we want to push to.