Upgrade/Install: Only show auto-update for themes that support the feature.

Similar to the changes for plugins in [48669], let's only show the UI for themes when updates are supported for that theme.

This brings the changes from [48688] to the 5.5 branch.

See #50280.
Props dd32.

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git-svn-id: https://core.svn.wordpress.org/branches/5.5@48460 1a063a9b-81f0-0310-95a4-ce76da25c4cd