Update requirements to match README

website/index.html

@@ -172,7 +172,7 @@ GitHubService service = retrofit.create(GitHubService.class);</pre>
               <pre class="prettyprint">
 implementation 'com.squareup.retrofit2:retrofit:<span class="version pln"><em>(insert latest version)</em></span>'
 </pre>
-              <p>Retrofit requires at minimum Java 7 or Android 2.3.</p>
+              <p>Retrofit requires at minimum Java 8+ or Android API 21+.</p>
 
               <h4>R8 / ProGuard</h4>
               <p>If you are using R8 or ProGuard add the options from <a href="https://github.com/square/retrofit/blob/master/retrofit/src/main/resources/META-INF/proguard/retrofit2.pro">this file</a>.</p>