Update dependency com.diffplug.spotless to v6.21.0 (#3119)

| datasource | package                                                   | from   | to     |
| ---------- | --------------------------------------------------------- | ------ | ------ |
| maven      | com.diffplug.spotless:com.diffplug.spotless.gradle.plugin | 6.20.0 | 6.21.0 |

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

gradle/libs.versions.toml

@@ -21,7 +21,7 @@ kotlinxSerialization = "1.6.0"
 mockWebServer = "4.11.0"
 retrofit = "2.9.0"
 retrofitKotlinxSerialization = "1.0.0"
-spotlessVersion = "6.20.0"
+spotlessVersion = "6.21.0"
 
 [libraries]
 coroutines-core = { module = "org.jetbrains.kotlinx:kotlinx-coroutines-core", version.ref = "coroutines" }