Rollback Backstop to v4.4.

[aaemnnosttv]

Summary

Addresses issue #2172

This PR is a follow-up to #2173 (merged) which didn't completely address the issue.

In #1865 we moved VRT to run on GitHub Actions. As part of this work, it was necessary to customize the Docker command that is used to run the tests, due to TTY complications on the Actions platform.

As a result, Backstop was upgraded to the latest version (5.x), however the necessary dockerCommandTemplate config argument has been available since v3.9.6

After digging into this a bit more on the Backstop issues, I found a few issues of users having similar problems with newer versions of Backstop.
See garris/BackstopJS#1156 (I also saw this locally)

Screenshot

Anecdotally, stability of Backstop JS seems to have become problematic starting around v4.5, hence this PR rolls it back to the latest version which seems to be more stable for most people).

Relevant technical choices

Checklist

• My code is tested and passes existing unit tests.
• My code has an appropriate set of unit tests which all pass.
• My code is backward-compatible with WordPress 4.7 and PHP 5.6.
• My code follows the WordPress coding standards.
• My code has proper inline documentation.
• I have added a QA Brief on the issue linked above.
• I have signed the Contributor License Agreement (see https://cla.developers.google.com/).

[tofumatt]

Read through the linked issue and seems like this should solve it.

(I wish there were a way to leave nice comments in the package.json warning future devs against upgrading this dependency later until it's fixed, but this is good. At least the commit message will point them here.)