Login and Registration: Update logo from blue to gray to match new design.
Developed in https://github.com/WordPress/wordpress-develop/pull/11026
Props juanfra, westonruter, jeffpaul, mukesh27, audrasjb, joedolson, dd32, davidbaumwald, fabiankaegy, huzaifaalmesbah, Joen, butterflymedia, noruzzaman, karmatosed, ozgursar, fcoveram, tusharaddweb, darshitrajyaguru97.
See #64308.
Fixes #64708.
Built from https://develop.svn.wordpress.org/trunk@61989
git-svn-id: http://core.svn.wordpress.org/trunk@61271 1a063a9b-81f0-0310-95a4-ce76da25c4cd
3ec4e4a641
