Use spotless for enforcing licenses

It's more verbose as the default license plugin doesn't support multiple
license headers. However, it also gives us some other goodies (namely
formatting Kotlin and removing unused imports), so worth doing.

buildSrc/settings.gradle.kts

@@ -0,0 +1,7 @@
+dependencyResolutionManagement {
+    versionCatalogs {
+        create("libs") {
+            from(files("../gradle/libs.versions.toml"))
+        }
+    }
+}