Change the priority of the search form & remove `float` so that the visual position of the form matches it's positioning in the DOM.
This sets the priority of the search form to an arbitrarily high value of `9999` to ensure it will generally be last in the DOM.
Props joedolson, sabernhardt, audrasjb.
Fixes#60685.
Built from https://develop.svn.wordpress.org/trunk@58215
git-svn-id: http://core.svn.wordpress.org/trunk@57678 1a063a9b-81f0-0310-95a4-ce76da25c4cd
