Date/Time: Remove `mysql2date()` usage in `get_feed_build_date()` to ensure...

Date/Time: Remove `mysql2date()` usage in `get_feed_build_date()` to ensure the output includes correct timezone offset.

With the changes in [45908], `mysql2date()` works correctly for all local time inputs, but should not be used for UTC time inputs.

Add a unit test.

Props Rarst, lisota.
Fixes #48675.
Built from https://develop.svn.wordpress.org/trunk@46756


git-svn-id: https://core.svn.wordpress.org/trunk@46556 1a063a9b-81f0-0310-95a4-ce76da25c4cd