[FrameworkBundle] Simplify "invokable" controllers as services

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | symfony/symfony-docs#3966

Simplifies the configuration when defining controllers as services that implement `__invoke`.

Old:

```yaml
my_route:
    path: /path
    defaults: { _controller: my.controller.service:__invoke }
```

New:

```yaml
my_route:
    path: /path
    defaults: { _controller: my.controller.service }
```