Update Kotlin versions and language settings

- Update Kotlin JVM plugin version to use the latest version
- Set Kotlin language version to 2.0 for all source sets
- Set JVM target to 17 for all Kotlin compile tasks
- Add compiler arguments for experimental APIs and opt-ins
- Configure test tasks to use JUnit Platform
- Update Kotlin JVM plugin version in the root `build.gradle.kts` file
- Set Kotlin language version to 2.0 in the root `build.gradle.kts` file

Ignores changes to imports and requires.

app-backend-jooq/build.gradle.kts

@@ -1,5 +1,13 @@
 plugins {
-    kotlin("jvm").version("1.9.0")
+    kotlin("jvm")
+}
+
+kotlin {
+    sourceSets.all {
+        languageSettings {
+            languageVersion = "2.0"
+        }
+    }
 }
 
 repositories {

app-backend/build.gradle.kts

@@ -1,7 +1,7 @@
 plugins {
     application
-    kotlin("jvm").version("1.9.0")
-    kotlin("plugin.serialization").version("1.9.0")
+    kotlin("jvm")
+    kotlin("plugin.serialization")
 }
 
 application {
@@ -14,15 +14,28 @@ repositories {
 }
 
 kotlin {
-    jvmToolchain(17)
+    sourceSets.all {
+        languageSettings {
+            languageVersion = "2.0"
+        }
+    }
 }
 
 tasks.withType<org.jetbrains.kotlin.gradle.tasks.KotlinCompile>().configureEach {
     kotlinOptions {
         jvmTarget = "17"
+        freeCompilerArgs = freeCompilerArgs + listOf(
+            "-opt-in=kotlinx.coroutines.ExperimentalCoroutinesApi",
+            "-opt-in=kotlinx.serialization.ExperimentalSerializationApi",
+            "-opt-in=io.ktor.server.locations.KtorExperimentalLocationsAPI",
+        )
     }
 }
 
+tasks.test {
+    useJUnitPlatform()
+}
+
 dependencies {
     implementation(kotlin("stdlib-jdk8"))
     implementation(kotlin("reflect"))

build.gradle.kts

@@ -1,6 +1,6 @@
 plugins {
     application
-    kotlin("jvm").version("1.9.0")
+    kotlin("jvm").version("1.9.20")
 }
 
 application {
@@ -17,6 +17,11 @@ tasks.test {
 
 kotlin {
     jvmToolchain(17)
+    sourceSets.all {
+        languageSettings {
+            languageVersion = "2.0"
+        }
+    }
 }
 
 tasks.withType<org.jetbrains.kotlin.gradle.tasks.KotlinCompile>().configureEach {