Add conflict for php-http/discovery:1.16.0

[fritzmg]

There is a regression in php-http/discovery:1.16.0 which was released an hour ago (php-http/message#152 (comment)).

[leofeyer]

Thank you @fritzmg.

[nicolas-grekas]

Please don't do that !

[nicolas-grekas]

The correct fix is two steps:

1. require php-http/message-factory directly, since you apparently need it
2. later on: figure out why you need php-http/message-factory and replace that need by PSR-17

[fritzmg]

Please don't do that !

We need to do that, otherwise existing and new installations will break.

[Toflar]

We will investigate and apply the right fix in later versions but for the time being, contao/conflicts is our way to make sure people can continue to install current versions.

[fritzmg]

The correct fix is two steps:

1. require php-http/message-factory directly, since you apparently need it
2. later on: figure out why you need php-http/message-factory and replace that need by PSR-17

@nicolas-grekas we don't use anything from php-http directly. php-http/message and php-http/discovery is installed via friendsofsymfony/http-cache. Apparently php-http/discovery then wants to automatically discover a HTTP client, however we already require symfony/http-client.