Add Kotlin support section

This is a prerequisite for dropping compatibility with Kotlin pre-2.x.

See assertj/assertj#3870.

Author: scordio

gradle.properties

@@ -6,7 +6,7 @@ version             = 1.0
 # Note that if --release is added then -target and -source are ignored.
 javacRelease = 8
 
-assertjVersion         = 3.27.3
+assertjVersion         = 3.27.6
 assertjJodaTimeVersion = 2.2.0
 assertjDbVersion       = 2.0.2
 ota4jVersion           = 1.2.0

src/docs/asciidoc/user-guide/assertj-core-quickstart.adoc

@@ -8,6 +8,11 @@ This guide is for the AssertJ Core module.
 
 AssertJ Core requires Java 8 or higher.
 
+[[assertj-core-kotlin]]
+===== Kotlin Support
+
+AssertJ Core is compatible with Kotlin 1.9 or higher.
+
 [[assertj-core-android]]
 ===== Android Support