Add `conflict` section in composer

[franmomu]

IMO there should be a conflict section in sync with require-dev (with aws/aws-sdk-php and guzzlehttp/guzzle):

Elastica/composer.json

Lines 26 to 31 in 72a4598

	"require-dev": {
	"aws/aws-sdk-php": "^3.155",
	"guzzlehttp/guzzle": "^6.3 || ^7.2",
	"phpunit/phpunit": "^8.5.8 || ^9.4",
	"symfony/phpunit-bridge": "^5.1.1"
	},

So we only allow to installed tested versions.

[ruflin]

Can you provide an example (or PR) on how this would look?