feat: Update internal Kotlin version to 2.1.21-embedded

This commit updates the primary Kotlin version used by kscript for its own
build and for the Gradle scripts it generates (e.g., for --idea and --package)
from 1.7.21 to 2.1.21-embedded.

Changes include:

1.  **`build.gradle.kts`:**
    *   The `kotlinVersion` property has been changed to "2.1.21-embedded".
    *   The `kotlin("jvm")` plugin version has been updated to "2.1.21-embedded".
    *   Dependencies on `org.jetbrains.kotlin:*` artifacts were already
        parameterized to use `kotlinVersion`, so they will automatically adopt
        the new version.

2.  **`GradleTemplates.kt`:**
    *   Verified that generated Gradle scripts for `--idea` and `--package`
        already use `KotlinVersion.CURRENT` to dynamically set their
        Kotlin plugin and `kotlin-script-runtime` versions. This ensures
        they will use the new "2.1.21-embedded" version.
    *   Updated `kotlin-stdlib` declarations within these templates to also
        explicitly use the dynamic `kotlinVersion` for consistency and clarity.

No other hardcoded references to the old Kotlin version were found in kscript's
core operational code that required changes for this update. Runtime version
information displayed to you (e.g., via `kscript --version`) should
dynamically reflect this new version through `BuildConfig.KOTLIN_VERSION`.

Author: google-labs-jules[bot]

build.gradle.kts

@@ -4,10 +4,10 @@ import org.jetbrains.kotlin.util.capitalizeDecapitalize.toLowerCaseAsciiOnly
 import java.time.ZoneOffset
 import java.time.ZonedDateTime
 
-val kotlinVersion: String = "1.7.21"
+val kotlinVersion: String = "2.1.21-embedded"
 
 plugins {
-    kotlin("jvm") version "1.7.21"
+    kotlin("jvm") version "2.1.21-embedded"
     application
     id("com.adarshr.test-logger") version "3.2.0"
     id("com.github.gmazzo.buildconfig") version "3.1.0"

src/main/kotlin/io/github/kscripting/kscript/code/GradleTemplates.kt

@@ -10,7 +10,7 @@ object GradleTemplates {
     fun createGradleIdeaScript(script: Script): String {
         val kotlinVersion = KotlinVersion.CURRENT
         val extendedDependencies = setOf(
-            Dependency("org.jetbrains.kotlin:kotlin-stdlib"),
+            Dependency("org.jetbrains.kotlin:kotlin-stdlib:$kotlinVersion"),
             Dependency("org.jetbrains.kotlin:kotlin-script-runtime:$kotlinVersion"),
             Dependency("io.github.kscripting:kscript-annotations:1.5.0"),
         ) + script.dependencies
@@ -42,7 +42,7 @@ object GradleTemplates {
 
         val kotlinVersion = KotlinVersion.CURRENT
         val extendedDependencies = setOf(
-            Dependency("org.jetbrains.kotlin:kotlin-stdlib"),
+            Dependency("org.jetbrains.kotlin:kotlin-stdlib:$kotlinVersion"),
             Dependency("org.jetbrains.kotlin:kotlin-script-runtime:$kotlinVersion")
         ) + script.dependencies