Update plugin buildkonfig to v0.21.0

| datasource | package                                                                 | from   | to     |
| ---------- | ----------------------------------------------------------------------- | ------ | ------ |
| maven      | com.codingfeline.buildkonfig:com.codingfeline.buildkonfig.gradle.plugin | 0.15.1 | 0.21.0 |

Author: renovate[bot]

gradle/libs.versions.toml

@@ -100,5 +100,5 @@ kotlin-parcelize = { id = "org.jetbrains.kotlin.plugin.parcelize", version.ref =
 kotlin-serialization = { id = "org.jetbrains.kotlin.plugin.serialization", version.ref = "kotlin" }
 
 ksp = { id = "com.google.devtools.ksp", version.ref = "ksp" }
-buildkonfig = { id = "com.codingfeline.buildkonfig", version = "0.15.1" }
+buildkonfig = { id = "com.codingfeline.buildkonfig", version = "0.21.0" }
 spotless = { id = "com.diffplug.gradle.spotless", version = "6.25.0" }