Validate the top level reserved words

Thanks to @arihantar we noticed that not all reserved words in the Gitlab::PathRegex::TOP_LEVEL_ROUTES are required to be there.

For example the word repository is never used in a top level route.

I think these are leftovers from before the nested groups era.

But we need to be careful when removing these, because the specs only take rails routes into account, and not the files hosted from the public & public/uploads directory.

This could be an easy win to free up some top-level namespaces for #26407 (moved)

cc @DouweM