[2.x] Style improvements

[jessarcher]

This PR applies the same changes to Jetstream that https://github.com/laravel/breeze/pull/177 applied to Breeze:

• It adds the missing dark text variant for the nav links on the Vue Welcome page that is present in the Blade version shipped with the skeleton.
• It removes the trailing underlined whitespace from the "Log in" link.
• It removes the styles that are baked into the Welcome page in favor of using those provided by Tailwind.

Before

After