Social Icons widget shows WordPress icon for WordPress.com RSS Feeds, not RSS icon

#### Steps to reproduce the issue

1. Add the Social Icons widget.
2. Add a WordPress.com RSS feed, like https://mmxtest.wordpress.com/feed/
3. Scroll down to the widget.
4. You'll see the problem.

#### What I expected
Here's what you see when you add any other RSS feed, like http://delta.jhuff.me/feed/

#### What happened instead
Here's what you see when you add an RSS feed from a site with a WordPress.com URL:

I was able to reproduce this both on a site running WordPress 5.5.3 and Jetpack 9.1, and on a WordPress.com site, using both the Twenty Twenty and Seedlet themes to confirm.

originally reported at https://wordpress.com/forums/topic/rss-feed-on-social-icons-widget/

Start by reproducing the Social Icons widget behavior with a WordPress.com RSS feed such as https://mmxtest.wordpress.com/feed/ and compare it with a regular RSS feed. Trace how the widget identifies each feed type; done means WordPress.com RSS feeds display the RSS icon rather than the WordPress icon, as shown in the expected result.

Written by the indexing model from the issue text.