Bump kotlinVersion from 1.8.20 to 1.9.21

Bumps `kotlinVersion` from 1.8.20 to 1.9.21.

Updates `org.jetbrains.kotlin.android` from 1.8.20 to 1.9.21

Updates `org.jetbrains.kotlin.kapt` from 1.8.20 to 1.9.21

Updates `org.jetbrains.kotlin.multiplatform` from 1.8.20 to 1.9.21

Updates `org.jetbrains.kotlin:kotlin-stdlib-jdk8` from 1.8.20 to 1.9.21
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](https://github.com/JetBrains/kotlin/compare/v1.8.20...v1.9.21)

---
updated-dependencies:
- dependency-name: org.jetbrains.kotlin.android
  dependency-type: direct:production
  update-type: version-update:semver-minor
- dependency-name: org.jetbrains.kotlin.kapt
  dependency-type: direct:production
  update-type: version-update:semver-minor
- dependency-name: org.jetbrains.kotlin.multiplatform
  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
...

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

build.gradle.kts

@@ -1,5 +1,5 @@
 plugins {
-    val kotlinVersion = "1.8.20"
+    val kotlinVersion = "1.9.21"
     id("com.android.application") version "8.1.4" apply (false)
     id("org.jetbrains.kotlin.android") version kotlinVersion apply (false)
     id("org.jetbrains.kotlin.kapt") version kotlinVersion apply (false)

uhabits-android/build.gradle.kts

@@ -95,7 +95,7 @@ android {
 
 dependencies {
     val daggerVersion = "2.48.1"
-    val kotlinVersion = "1.8.20"
+    val kotlinVersion = "1.9.21"
     val kxCoroutinesVersion = "1.7.3"
     val ktorVersion = "1.6.8"
     val espressoVersion = "3.5.1"

uhabits-server/build.gradle.kts

@@ -38,7 +38,7 @@ application {
 
 dependencies {
     val ktorVersion = "1.6.8"
-    val kotlinVersion = "1.8.20"
+    val kotlinVersion = "1.9.21"
     val logbackVersion = "1.4.9"
     implementation("org.jetbrains.kotlin:kotlin-stdlib-jdk8:$kotlinVersion")
     implementation("io.ktor:ktor-server-netty:$ktorVersion")