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

See #25834.

git-svn-id: https://develop.svn.wordpress.org/trunk@30750 602fd350-edb4-49c9-b593-d223f7449a82
Remove `<code>` tags from translatable string in `WP_Date_Query`.

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

Props ramiy.
Fixes #30612.

git-svn-id: https://develop.svn.wordpress.org/trunk@30751 602fd350-edb4-49c9-b593-d223f7449a82