Doc API level 21 as minimum version for Gson 2.14.0

The minimum API level for the `check-android-compatibility` check had
been increased to 23, but it seems there was no usage from that API level.

Author: Marcono1234

README.md

@@ -70,7 +70,6 @@ see [`GsonBuilder.disableJdkUnsafe()`](https://javadoc.io/doc/com.google.code.gs
 #### Minimum Android API level
 
 - Gson 2.15.0 and newer: API level 24
-- Gson 2.14.0 and newer: API level 23
 - Gson 2.11.0 and newer: API level 21
 - Gson 2.10.1 and older: API level 19