(Minor) Update README.md and BUILD.md (#767)

We were oddly missing the JDK requirement.

Author: docker-lordvf6ik

BUILD.md

@@ -7,6 +7,7 @@ It can run on many different platforms, including mobile devices (iOS and Androi
 
 On all platforms:
 
+- [OpenJDK 21](https://adoptium.net/temurin/releases/?package=jdk&version=21)
 - [Docker](https://www.docker.com/) 18.03 or newer is required if you want to run all tests
 
 For iOS, you need to have [Xcode](https://developer.apple.com/xcode/) and [Homebrew](https://brew.sh/) installed, and then:

README.md

@@ -1,4 +1,4 @@
-[![Kotlin](https://img.shields.io/badge/Kotlin-1.9.23-blue.svg?style=flat&logo=kotlin)](http://kotlinlang.org)
+[![Kotlin](https://img.shields.io/badge/Kotlin-2.1.10-blue.svg?style=flat&logo=kotlin)](http://kotlinlang.org)
 [![Maven Central](https://img.shields.io/maven-central/v/fr.acinq.lightning/lightning-kmp)](https://search.maven.org/search?q=g:fr.acinq.lightning%20a:lightning-kmp*)
 ![Github Actions](https://github.com/ACINQ/lightning-kmp/actions/workflows/test.yml/badge.svg)
 [![License](https://img.shields.io/badge/license-Apache%202.0-blue.svg)](LICENSE)