Bump to Kotlin 1.5 (#175)

* Bump to Kotlin 1.5

Kotlin 1.5 is out, but kotlin-logging still being on 1.4 causes
dependency conflicts in my project. With this change, I bump the
Kotlin version to 1.5.

https://blog.jetbrains.com/kotlin/2021/05/kotlin-1-5-0-released/

* Update build.gradle.kts

Co-authored-by: Julien Lehuen <julien.lehuen@imc.com>
Co-authored-by: Ohad Shai <ohadshai@gmail.com>

build.gradle.kts

@@ -1,6 +1,6 @@
 plugins {
-    kotlin("multiplatform") version "1.4.31"
-    id("org.jetbrains.dokka") version "1.4.32"
+    kotlin("multiplatform") version "1.5.21"
+    id("org.jetbrains.dokka") version "1.5.0"
     `maven-publish`
     id("io.github.gradle-nexus.publish-plugin") version "1.1.0"
     signing