Thanks again @AmauryPi! I finally got around to updating this and it works great! The feeds validate. But, there is a warning which is easy to fix. For whatever reason, the feed validators don't like the link to the feed to end in a slash. It likes the other links to end in a slash, but not the feed URL. I have no idea why... and it could even be that the feed validator is wrong. But, to get rid of that warning, here's the fix...
The validator gives me a warning with:
<link href="https://mysite.com/atom/" rel="self" />
If I remove the slash at the end of the URL so that it looks like the following that warning goes away:
<link href="https://mysite.com/atom" rel="self" />
Remove the slash in AbstractFeedController.php on line 124:
'self_link' => rtrim($request->getUri(), " \t\n\r\0\v/") . "/",
'self_link' => rtrim($request->getUri(), " \t\n\r\0\v/"),