…updates in the default user agents (javiereguiluz)

This PR was merged into the 6.3 branch.

Discussion
----------

[BrowserKit][HttpClient] [BrowserKit, HttpClient] Minor updates in the default user agents

| Q             | A
| ------------- | ---
| Branch?       | 6.3
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       | -
| License       | MIT
| Doc PR        | -

After merging #50053 I realized that some of my changes were not really needed.

According to the spec, the user agent _"consists of one or more product identifiers, each followed by zero or more comments"_

For example this user agent:

```
Mozilla/5.0 (iPad; U; CPU OS 3_2_1 like Mac OS X; en-us) AppleWebKit/531.21.10 (KHTML, like Gecko) Mobile/7B405
```

`Mozilla/5.0` is the product and version
`AppleWebKit/531.21.10` is a subproduct and its own version
In both cases the version is optional.

So, our user agent `Symfony BrowserKit` is correct:

* `Symfony` is the product, without version (completely legal)
* `BrowserKit` is the subproduct, and omits its version (legal too)

The real issue was with user agents like `Symfony HttpClient/Amp`, because `/Amp` would be interpreted as the version of the `HttpClient`, which is wrong (the version is `6.2`, `6.3`, etc.). The `Amp` text is a detail about a product, so it's common to wrap it in parenthesis.

So, sorry for the noise and I think this time this is fully correct. Thanks.

Commits
-------

e4d3203 [BrowserKit, HttpClient] Minor updates in the default user agents