Posts, Post Types: Pass the post type to `get_lastpostdate()` in `get_lastpostmodified()`.

This ensures that the correct values are being compared when retrieving the result for custom post types.

Additionally, pass the `$post_type` parameter to `get_lastpostdate` and `get_lastpostmodified` filters.

Props mikaumoto, munyagu, donmhico, johnbillion, SergeyBiryukov.
Fixes #47777.
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git-svn-id: https://core.svn.wordpress.org/trunk@48393 1a063a9b-81f0-0310-95a4-ce76da25c4cd