Resolve some deprecations and remove igorw/get-in dependency (#1257)

* Remove `igorw/get-in` dependency to simplify code

* Fix phpunit deprecation

* Simplify phpunit's `->will($this->returnValue())` with `->willReturn()`

* Simplify phpunit's `->will($this->returnCallback())` with `->willReturnCallback()`

* Fix deprecated mock builder `setMethds` with `createPartialMock`

* Remove deprecated phpstan `checkGenericClassInNonGenericObjectType` option

* Bump phpunit to 9.6

---------

Co-authored-by: Jonathan Beliën <jbelien@users.noreply.github.com>

Author: norkunas

composer.json

@@ -24,7 +24,7 @@
         "nyholm/psr7": "^1.0",
         "php-http/message": "^1.0",
         "php-http/mock-client": "^1.0",
-        "phpunit/phpunit": "^9.5",
+        "phpunit/phpunit": "^9.6.11",
         "symfony/stopwatch": "~2.5 || ~5.0"
     },
     "extra": {