Use php-http/discovery": "^1.15

[cleptric]

From #1459

This is a follow up of #1429
Since this PR and our discussion at SymfonyCon, I've been working on moving those ideas to "php-http/discovery", and v1.15.0 has just been tagged 🎉
By bumping to v1.15.0, we keep compatibility with PHP >= 7.1 and composer v1 & v2, but we also benefit from:

• Add generic Psr17Factory php-http/discovery#209
• Add composer plugin to auto-install missing implementations php-http/discovery#208
  This allows simplifying the dependencies of sentry-php since we can now move guzzlehttp/psr7 from require to require-dev.

[cleptric]

Still required inside GuzzleTracingMiddleware, but so is GuzzleHttp\Exception\RequestException, which we do not require. Hence people need to require composer require guzzlehttp/guzzle already, so removing it is fine imo.