[php] 3.0 - composer.json should allow guzzlehttp/guzzle ^7.0

[oprudkyi]

Description

The blocking change already fixed in b99fea7
so composer.json should be updated for 7.0 support

Swagger-codegen version

3.x

Related issues/PRs

#11104

Suggest a fix/enhancement

https://github.com/swagger-api/swagger-codegen/blob/master/modules/swagger-codegen/src/main/resources/php/composer.mustache

-        "guzzlehttp/guzzle": "^6.2"
+        "guzzlehttp/guzzle": "^6.2|^7.0"

[oprudkyi]

seems we are using 3.x version, moving to correct repository