Themes: Include theme name in available update messages, for better...

Themes: Include theme name in available update messages, for better accessibility and consistency with other similar messages.

Follow-up to [48652-48654].

See #48491.
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git-svn-id: https://core.svn.wordpress.org/trunk@48421 1a063a9b-81f0-0310-95a4-ce76da25c4cd