drop android 6 (planning discussion)

We already warned users that android 6 will stop working #1264

Currently our minSDK version is 23. Using minSdkVersion 24 gives us these:

API Java 8 | minSdkVersion compatible
-- | --
java.lang.annotation.Repeatable | API nivel 24 o versiones posteriores
AnnotatedElement.getAnnotationsByType(Class) | API nivel 24 o versiones posteriores
java.util.stream | API nivel 24 o versiones posteriores
java.lang.FunctionalInterface | API nivel 24 o versiones posteriores
java.lang.reflect.Method.isDefault() | API nivel 24 o versiones posteriores
java.util.function | API nivel 24 o versiones posteriores

(current api level is 29 i.e. Android 10)

@AW-STJ @James-Sangalli I opt for making minsdk 24 for downstream products that you are managing, because that saves us the trouble of upping it later, which eventually will happen.