chore(deps): update plugin org.jetbrains.kotlin.multiplatform to v2.1.21 (#352)

This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| org.jetbrains.kotlin.multiplatform | `2.1.20` -> `2.1.21` |
[![age](https://developer.mend.io/api/mc/badges/age/maven/org.jetbrains.kotlin.multiplatform:org.jetbrains.kotlin.multiplatform.gradle.plugin/2.1.21?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|
[![adoption](https://developer.mend.io/api/mc/badges/adoption/maven/org.jetbrains.kotlin.multiplatform:org.jetbrains.kotlin.multiplatform.gradle.plugin/2.1.21?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|
[![passing](https://developer.mend.io/api/mc/badges/compatibility/maven/org.jetbrains.kotlin.multiplatform:org.jetbrains.kotlin.multiplatform.gradle.plugin/2.1.20/2.1.21?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|
[![confidence](https://developer.mend.io/api/mc/badges/confidence/maven/org.jetbrains.kotlin.multiplatform:org.jetbrains.kotlin.multiplatform.gradle.plugin/2.1.20/2.1.21?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|

---

### Configuration

📅 **Schedule**: Branch creation - "every 1 week on saturday" (UTC),
Automerge - At any time (no schedule defined).

🚦 **Automerge**: Enabled.

♻ **Rebasing**: Whenever PR is behind base branch, or you tick the
rebase/retry checkbox.

🔕 **Ignore**: Close this PR and you won't be reminded about this update
again.

---

- [ ] <!-- rebase-check -->If you want to rebase/retry this PR, check
this box

---

This PR was generated by [Mend Renovate](https://mend.io/renovate/).
View the [repository job
log](https://developer.mend.io/github/typesafegithub/github-actions-typing).

<!--renovate-debug:eyJjcmVhdGVkSW5WZXIiOiI0MC4xMS4xOCIsInVwZGF0ZWRJblZlciI6IjQwLjExLjE4IiwidGFyZ2V0QnJhbmNoIjoibWFpbiIsImxhYmVscyI6W119-->

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

build.gradle.kts

@@ -3,7 +3,7 @@ import org.jetbrains.kotlin.gradle.targets.js.nodejs.NodeJsRootPlugin.Companion.
 import org.jetbrains.kotlin.gradle.targets.js.yarn.yarn
 
 plugins {
-    kotlin("multiplatform") version "2.1.20"
+    kotlin("multiplatform") version "2.1.21"
     kotlin("plugin.serialization") version "2.1.20"
     id("io.kotest.multiplatform") version "6.0.0.M3"
     distribution