update readme supported android versions

We support Android 14 (API 34) for a while but this readme was never
updated. Also since OneSignal-Android-SDK 5.0.0 we only support
Android 5.0 (API 21).

Author: jkasten2

README.md

@@ -28,4 +28,4 @@ For account issues and support please contact OneSignal support from the [OneSig
 To make things easier, we have published demo projects in the `/Examples` folder of this repository.
 
 #### Supports:
-* Tested from Android 4.1.0 (API level 16) to Android 12 (31)
+* Tested from Android 5.0 (API level 21) to Android 14 (34)