Media Library: Set max value of columns to 12.

fixes #29275.
Built from https://develop.svn.wordpress.org/trunk@29547


git-svn-id: http://core.svn.wordpress.org/trunk@29323 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Media Grid: Don't hide error messages behind the toolbar.

fixes #29141.
Built from https://develop.svn.wordpress.org/trunk@29548


git-svn-id: http://core.svn.wordpress.org/trunk@29324 1a063a9b-81f0-0310-95a4-ce76da25c4cd