refactor(core): Fix Kotlin language and api version to 2.1 in build config (#620)

## Fix Kotlin language and api version to 2.1

- Remove language constructs not supported in Kotlin 2.1

<!-- Provide a brief summary of your changes -->

## Motivation and Context
To ensure binary compatibility

## How Has This Been Tested?
Regression tests

## Breaking Changes
<!-- Will users need to update their code or configurations? -->

## Types of changes
<!-- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update
- [x] Build update

## Checklist
<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->
- [x] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [x] My code follows the repository's style guidelines
- [x] New and existing tests pass locally
- [ ] I have added appropriate error handling
- [ ] I have added or updated documentation as needed

## Additional context
<!-- Add any other context, implementation notes, or design decisions
-->

buildSrc/src/main/kotlin/mcp.multiplatform.gradle.kts

@@ -5,6 +5,7 @@ import org.jetbrains.kotlin.gradle.ExperimentalWasmDsl
 import org.jetbrains.kotlin.gradle.dsl.ExplicitApiMode
 import org.jetbrains.kotlin.gradle.dsl.JvmDefaultMode
 import org.jetbrains.kotlin.gradle.dsl.JvmTarget
+import org.jetbrains.kotlin.gradle.dsl.KotlinVersion
 
 plugins {
     kotlin("multiplatform")
@@ -15,6 +16,8 @@ plugins {
 kotlin {
 
     compilerOptions {
+        languageVersion = KotlinVersion.KOTLIN_2_1
+        apiVersion = KotlinVersion.KOTLIN_2_1
         freeCompilerArgs =
             listOf(
                 "-Wextra",