Remove support for Drush as a global project

[weitzman]

Folks can use Drush Launcher or any of the alternatives described there.

[weitzman]

Remove RedispatchToSiteLocal class and its call in \Drush\Preflight\Preflight::preflight

[greg-1-anderson]

@weitzman Drush launcher doesn't support aliases, and it seems that adding alias support there isn't gaining traction.

Instead of removing redispatchToSiteLocal, maybe instead we should only simplify the bootstrap logic, so that Drush can only bootstrap a Drupal site that it shares a vendor directory with.

This would allow us to retain the global Drush as an alias-aware launcher, and would also give us a place to put archive:restore.

[weitzman]

That makes sense, I think.

[weitzman]

See #5521. drush can still be used as a global drush (unsupported) but a site-local version is still required.