Site Health: Only display the warning about the `post_max_size` PHP value when...

Site Health: Only display the warning about the `post_max_size` PHP value when it's lower than `upload_max_filesize`.

Previously, the warning was displayed even if the value was higher than `upload_max_filesize`, which made the report inaccurate, as these values don't necessarily have to match.

Props oakesjosh, jeroenrotty, desrosj, Clorith.
Merges [48800] and [48801] to the 5.5 branch.
Fixes #50945.
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git-svn-id: https://core.svn.wordpress.org/branches/5.5@48568 1a063a9b-81f0-0310-95a4-ce76da25c4cd