[FrameworkBundle] Simplify "invokable" controllers as services
feature #11193 [FrameworkBundle] Simplify "invokable" controllers as services (kbond)

This PR was submitted for the 2.3 branch but it was merged into the 2.4-dev branch instead (closes #11193).

Discussion
----------

[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 }
```

Commits
-------

e712e89 [FrameworkBundle] Simplify "invokable" controllers as services