bump android API level to 21 for 2020 (#2233)

Author: bitsandfoxes

test/Scripts.Integration.Test/Editor/Builder.cs

@@ -86,6 +86,12 @@ public static void BuildIl2CPPPlayer(BuildTarget target, BuildTargetGroup group,
             Debug.Log($"Builder: Creating output directory at '{outputDir}'");
             Directory.CreateDirectory(outputDir);
 
+#if UNITY_2020_3
+            // The default for 2020.3 is 19. There will be no further updates to 2020.3.
+            Debug.Log("Builder: Raising the minSdkVersion to 21");
+            PlayerSettings.Android.minSdkVersion = AndroidSdkVersions.AndroidApiLevel21;
+#endif
+            
             Debug.Log("Builder: Enabling minify");
 #if UNITY_2020_1_OR_NEWER
             PlayerSettings.Android.minifyDebug = PlayerSettings.Android.minifyRelease = true;