Use `wp_checkdate()` when checking date validity in `WP_Date_Query`.

See #25834.
Built from https://develop.svn.wordpress.org/trunk@30750


git-svn-id: http://core.svn.wordpress.org/trunk@30740 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Remove `<code>` tags from translatable string in `WP_Date_Query`.

This was missed in [30300]. See #25834.

Props ramiy.
Fixes #30612.
Built from https://develop.svn.wordpress.org/trunk@30751


git-svn-id: http://core.svn.wordpress.org/trunk@30741 1a063a9b-81f0-0310-95a4-ce76da25c4cd