Gravatar: Default image requests to the Gravatar service will result in a warning

[henrywright]

Gravatar is asking us to use urlencode() when requesting default images. For example

urlencode( 'https://example.com/images/avatar.jpg' )

Source: https://docs.gravatar.com/general/images/

urlencode() usage results in the following warning:

urlencode() should only be used when dealing with legacy applications rawurlencode() should now be used instead. See https://www.php.net/function.rawurlencode and http://www.faqs.org/rfcs/rfc3986.html

Related: #1234