core/social-link component doesn't render accessible markup

[0aveRyan]

Describe the bug
The anchor markup rendered by core/social-link isn't accessible. Anchors need text to describe the links to a screen reader.

Render method

To reproduce
Steps to reproduce the behavior:

1. Go to Block Editor
2. Insert the Social Links block
3. Click Preview to see rendered block markup
4. Inspect Anchors or run Google Lighthouse, which will flag the issue as Links do not have a discernible name, multiplied by the number of inserted anchors.

Expected behavior
In the current implementation, anchor elements simply wrap SVGs of each social brand. The name of each site is already available and used to add a unique CSS class, so the fix should be to a fairly light lift to add a default, descriptive aria-label="{BRAND} logo" to the <a>.

However, we might also consider adding a custom override for the AIRA label in core/social-link. If a blog post was created identifying social profiles for 10 Fast-Food Brands That Have Fire Social Accounts, it's possible there could be many instances of the Social Links block. Having a screen reader read "Twitter logo" 10+ times throughout the document isn't nearly as useful as...

• "Twitter list for 10 Fast-Food Brands That Have Fire Social Accounts"
• "Twitter profile for Shake Shack",
• "Twitter profile for Popeyes"
• and "Twitter profile for AwesomeFastFoodReviews.com" [the current site].

[0aveRyan]

@gziolo I've opened a PR to address this in #18651