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
-->

kotlin-sdk-core/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/types/request.kt

@@ -16,9 +16,9 @@ import kotlin.jvm.JvmInline
 public value class RequestMeta(public val json: JsonObject) {
     public val progressToken: ProgressToken?
         get() = json["progressToken"]?.let { element ->
-            when (element) {
-                is JsonPrimitive if (element.isString) -> ProgressToken(element.content)
-                is JsonPrimitive if (element.longOrNull != null) -> ProgressToken(element.long)
+            when {
+                element is JsonPrimitive && element.isString -> ProgressToken(element.content)
+                element is JsonPrimitive && element.longOrNull != null -> ProgressToken(element.long)
                 else -> null
             }
         }

kotlin-sdk-core/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/types/serializers.kt

@@ -414,9 +414,9 @@ internal object JSONRPCMessagePolymorphicSerializer :
  * Supports both string and number IDs.
  */
 internal object RequestIdPolymorphicSerializer : JsonContentPolymorphicSerializer<RequestId>(RequestId::class) {
-    override fun selectDeserializer(element: JsonElement): DeserializationStrategy<RequestId> = when (element) {
-        is JsonPrimitive if (element.isString) -> RequestId.StringId.serializer()
-        is JsonPrimitive if (element.longOrNull != null) -> RequestId.NumberId.serializer()
+    override fun selectDeserializer(element: JsonElement): DeserializationStrategy<RequestId> = when {
+        element is JsonPrimitive && element.isString -> RequestId.StringId.serializer()
+        element is JsonPrimitive && element.longOrNull != null -> RequestId.NumberId.serializer()
         else -> throw SerializationException("Invalid RequestId type: $element")
     }
 }