Modifies gravatar image URLs to always use the HTTPS version from secure.gravatar.com.
Gravatar now redirects HTTP image requests to their HTTPS equivalent, resulting in redirects for sites running over an HTTP connection (`is_ssl() === false`). Since the introduction of HTTP/2 the use of sub-domains for different hashes ([1-3].gravatar.com) now represents a performance hinderance rather than improvement.
The scheme passed to `get_avatar_data()` is now ignored for the generation of Gravatar URLs but the setting retained to avoid introducing bugs for sites using either local avatars or third party providers.
Props neoxx, SergeyBiryukov, sippis, peterwilsoncc, mukesh27, costdev, dd32.
Fixes#37454.
Built from https://develop.svn.wordpress.org/trunk@58822
git-svn-id: http://core.svn.wordpress.org/trunk@58218 1a063a9b-81f0-0310-95a4-ce76da25c4cd