Make WP_Widget_Media::get_instance_schema() public

[westonruter]

Currently the WP_Widget_Media::get_instance_schema() method is protected for the sake of:

This is protected because it may become part of WP_Widget eventually.

See #35574 (Add REST API JSON schema information to WP_Widget).

The idea was that it could be made protected while we wait for #35574 to land. Nevertheless, if that ticket results in get_instance_schema becoming public then any existing media widget extensions in plugins which retain protected visibility will then fatal error, which can be seen here: https://3v4l.org/j6emh

So I think we should just go ahead and make it public now, especially if we can validate that #35574 is indeed desired and should become part of WP_Widget itself.