Meta: Reorder the get_metadata_default() signature to match get_metadata().

The order of parameters in `get_metadata_default()` did not match the signature of `get_metadata()`. This could be confusing for developers who are familiar with the existing metadata API.

Fixes #43941.
Props SergeyBiryukov, spacedmonkey, johnjamesjacoby.

Built from https://develop.svn.wordpress.org/trunk@48502


git-svn-id: https://core.svn.wordpress.org/trunk@48264 1a063a9b-81f0-0310-95a4-ce76da25c4cd