Site Health: Rename `upload_max` array key in file upload checks to...

Site Health: Rename `upload_max` array key in file upload checks to `max_effective_size` for clarity.

Follow-up to [48535].

See #50038.
Built from https://develop.svn.wordpress.org/trunk@48544


git-svn-id: https://core.svn.wordpress.org/trunk@48306 1a063a9b-81f0-0310-95a4-ce76da25c4cd