Plugin deletion results must be stored in an option instead of a transient to avoid the transient being removed on shutdown

[kkmuffme]

​https://github.com/WordPress/wordpress-develop/blob/trunk/src/wp-admin/plugins.php#L431

When using an external object cache, if any plugin hooks wp_cache_flush on shutdown hook/register shutdown function, this transient will be removed and the plugin delete result is never shown to the user.

The result should be stored in an option instead of a transient to fix this problem.

You could argue that you shouldnt call cache flush on shutdown, however this is often the only way to fix compatibility issues where plugins do not remove their data from cache on uninstall and you are left with a broken site after uninstalling.