Symfony version(s) affected: 4.4.0

Description
Today I tried to composer update our application dependencies in a released version of our software. The update brought in the 4.4 version of symfony/routing. This version contains a BC break, because it removes the old syntax for defining a custom route loader service with 1 colon (https://symfony.com/doc/3.4/routing/custom_route_loader.html#loading-routes-with-a-custom-service).

Invalid resource "legacy.route_loader:loadRoutes" passed to the "service" route loader: use the format "object_id::method" or "object_id" if your object class has an "__invoke" method in legacy.route_loader:loadRoutes (which is being imported from "XXXBundle/Resources/config/routes.yaml"). Make sure there is a loader supporting the "service" type.

I did some digging and I think the source is this commit symfony/routing@314fecb#diff-601db0a084e4f4006fa48555089f7bf9.

Our used composer version constraint is ^4.3

How to reproduce
Define a custom service to load routes with the old syntax with one colon:
https://symfony.com/doc/3.4/routing/custom_route_loader.html#loading-routes-with-a-custom-service
Which was deprecated with 4.2 I think: https://symfony.com/doc/4.2/routing/custom_route_loader.html#loading-routes-with-a-custom-service

legacy:
    resource: legacy.route_loader:loadRoutes
    type: service