[Routing] Remove usage of deprecated _scheme requirement

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #8898, #8176
| License       | MIT
| Doc PR        | 

I removed all usages of the deprecated _scheme requirement inside the Routing Component.
Most parts were pretty easy and after multiple refactorings I came up with the solution to have a Route::hasScheme() method and check against this method.

I'm not sure if the change in UrlGenerator counts as BC break. If so I can change that. (see comment on change)

I also adjusted all tests that test '_scheme' to also check the new schemes-requirement.