use v3 API for github

The previously used beta API has become stable and declared as v3.

Using beta will return unexpected results that are not in sync with the API
documentation.

Author: splitbrain

src/OAuth/OAuth2/Service/GitHub.php

@@ -203,7 +203,7 @@ protected function getExtraOAuthHeaders()
      */
     protected function getExtraApiHeaders()
     {
-        return ['Accept' => 'application/vnd.github.beta+json'];
+        return ['Accept' => 'application/vnd.github.v3+json'];
     }
 
     /**

tests/Unit/OAuth2/Service/GitHubTest.php

@@ -216,6 +216,6 @@ public function testGetExtraApiHeaders(): void
         $headers = $service->request('https://pieterhordijk.com/my/awesome/path');
 
         self::assertArrayHasKey('Accept', $headers);
-        self::assertSame('application/vnd.github.beta+json', $headers['Accept']);
+        self::assertSame('application/vnd.github.v3+json', $headers['Accept']);
     }
 }