Multisite: Use 'warning' styles for 'Caution' and 'Warning' messages during installation

[dlh]

When installing a multisite network through the admin UI, there are (at least) two messages shown to the user that are styled using the updated CSS class but convey cautionary messages:

• "Warning: Please deactivate your plugins before enabling the Network feature."
• "Caution: We recommend you back up your existing wp-config.php file." (or variants of this)

Given the purpose of these messages, the warning styles might be more appropriate. The attached patch would make that switch; before-and-after screenshots are also attached to illustrate one of the two instances.

[johnjamesjacoby]

+1 from me

[SergeyBiryukov]

In 47067:

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

Props dlh.
Fixes #49186.