Update dependencies and JVM version

Upgraded Kotlin to 2.0.21, kotlinx-serialization to 1.8.1, and JVM version to 21. Adjusted `.gitignore` to include `.kotlin`. These updates ensure compatibility with latest standards and improve project maintainability.

Author: meanmail

.gitignore

@@ -105,3 +105,4 @@ fabric.properties
 token.txt
 .intellijPlatform
 .idea/ruff.xml
+.kotlin

gradle.properties

@@ -6,7 +6,7 @@ org.gradle.configuration-cache=true
 # Enable Gradle Build Cache -> https://docs.gradle.org/current/userguide/build_cache.html
 org.gradle.caching=true
 # https://plugins.jetbrains.com/docs/intellij/build-number-ranges.html#intellij-platform-based-products-of-recent-ide-versions
-jvmVersion=17
+jvmVersion=21
 #
 # Plugin
 group=dev.meanmail

gradle/libs.versions.toml

@@ -1,12 +1,12 @@
 [versions]
 # libraries
 junit = "4.13.2"
-kotlinxSerialization = "1.6.3"
+kotlinxSerialization = "1.8.1"
 
 # plugins
 intelliJPlatform = "2.7.0"
 # https://plugins.jetbrains.com/docs/intellij/using-kotlin.html#kotlin-standard-library
-kotlin = "1.9.25"
+kotlin = "2.0.21"
 
 [libraries]
 junit = { group = "junit", name = "junit", version.ref = "junit" }