Networks and Sites: Use `warning` styles for 'Caution' and 'Warning' messages...

Networks and Sites: Use `warning` styles for 'Caution' and 'Warning' messages during Multisite installation.

Props dlh.
Fixes #49186.
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git-svn-id: https://core.svn.wordpress.org/trunk@46867 1a063a9b-81f0-0310-95a4-ce76da25c4cd