Post Thumbnails: Make sure `get_post_thumbnail_id()` returns an integer, to...

Post Thumbnails: Make sure `get_post_thumbnail_id()` returns an integer, to match the documented return value.

This makes the function more consistent with `get_the_ID()` or `wp_get_post_parent_id()`, both returning an integer.

Props 0v3rth3d4wn.
Fixes #40096.
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git-svn-id: https://core.svn.wordpress.org/trunk@46960 1a063a9b-81f0-0310-95a4-ce76da25c4cd