Plugins: Move the "Successfully installed the plugin" string to the correct...

Plugins: Move the "Successfully installed the plugin" string to the correct `Plugin_Upgrader` method.

The string refers to the installation, and belongs in the `::install_strings()` method, not in `::upgrade_strings()`.

Props sswebster, nateinaction.
Reviewed by whyisjake, SergeyBiryukov.
Merges [48734] to the 5.5 branch.
Fixes #50837.
Built from https://develop.svn.wordpress.org/branches/5.5@48735


git-svn-id: https://core.svn.wordpress.org/branches/5.5@48497 1a063a9b-81f0-0310-95a4-ce76da25c4cd