Upgrade/Install: Ensure the proper count of plugins that have auto-update enabled.

As certain plugins may not support updates, this count needs to accurately reflect that in the navbar.

This brings the changes from [48703] to the WordPress 5.5 branch.

Fixes 50808.
Props knutsp, audrasjb, pbiron, dd32, apedog, afragen, chriscct7, garyc40, whyisjake, SergeyBiryukov.

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git-svn-id: https://core.svn.wordpress.org/branches/5.5@48466 1a063a9b-81f0-0310-95a4-ce76da25c4cd