Bump kotlin_version from 2.0.21 to 2.1.0

Bumps `kotlin_version` from 2.0.21 to 2.1.0.

Updates `org.jetbrains.kotlin.plugin.serialization` from 2.0.21 to 2.1.0
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/v2.1.0/ChangeLog.md)
- [Commits](JetBrains/kotlin@v2.0.21...v2.1.0)

Updates `org.jetbrains.kotlin:kotlin-stdlib-jdk8` from 2.0.21 to 2.1.0
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/v2.1.0/ChangeLog.md)
- [Commits](JetBrains/kotlin@v2.0.21...v2.1.0)

Updates `org.jetbrains.kotlin:kotlin-reflect` from 2.0.21 to 2.1.0
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/v2.1.0/ChangeLog.md)
- [Commits](JetBrains/kotlin@v2.0.21...v2.1.0)

Updates `org.jetbrains.kotlin:kotlin-gradle-plugin` from 2.0.21 to 2.1.0
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/v2.1.0/ChangeLog.md)
- [Commits](JetBrains/kotlin@v2.0.21...v2.1.0)

---
updated-dependencies:
- dependency-name: org.jetbrains.kotlin.plugin.serialization
  dependency-type: direct:production
  update-type: version-update:semver-minor
- dependency-name: org.jetbrains.kotlin:kotlin-stdlib-jdk8
  dependency-type: direct:production
  update-type: version-update:semver-minor
- dependency-name: org.jetbrains.kotlin:kotlin-reflect
  dependency-type: direct:production
  update-type: version-update:semver-minor
- dependency-name: org.jetbrains.kotlin:kotlin-gradle-plugin
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

biz.ganttproject.mxgraph/build.gradle

@@ -1,5 +1,5 @@
 buildscript {
-    ext.kotlin_version = '2.0.21'
+    ext.kotlin_version = '2.1.0'
     repositories {
         mavenCentral()
     }

build.gradle

@@ -29,7 +29,7 @@ allprojects {
     ext {
         libDir = 'lib'
         mvnDir = 'lib/mvn'
-        kotlin_version = "2.0.21"
+        kotlin_version = "2.1.0"
         java_version = "21"
         javaExportOptions = [
             '--add-exports', 'javafx.controls/com.sun.javafx.scene.control.behavior=ALL-UNNAMED',

ganttproject-tester/build.gradle

@@ -1,5 +1,5 @@
 buildscript {
-    ext.kotlin_version = '2.0.21'
+    ext.kotlin_version = '2.1.0'
     repositories {
         mavenCentral()
     }