[2.1][Process] Fix Process timeout

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #6976
| License       | MIT

This PR fixes timeout in the process component.
Timeout can now be float values. The precision of the timeout is given by the `Process::TIMEOUT_PRECISION` constant.

Note : The timeout works when running the process with the `Process::run` method. When using the `Process::start` method, the `Process::checkTimeout` method should be regularly triggered to perform a check.

example : 

```php
$process->start();

while ($condition) {
    // some stuff
    $process->checkTimeout(); // throws an Symfony\Process\Exception\RuntimeException if timeout is reached
    usleep(1000);
}
```