Profiler activation

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #7064, #7071
| License       | MIT
| Doc PR        | symfony/symfony-docs#2565

As stated in #7071, there is no way to disable the profiler completely. Even when the `enabled` flag is set to `false`, the profiler is still registered but the data collectors are not activated.

Now, when `enabled` is `false`, the profiler is disabled. To get the old `false` behavior, you now need to set `enabled` to `true` and set the new `collect` flag to `false`.

Todo:

 - [x] update docs
 - [x] update Symfony SE -- not needed