Prepare version bump of spotless

spotless v7 fails on Gradle with some `SerializationException`, moving the `callable` works around this issue (as [mentioned here](diffplug/spotless#2387)).

Also removing the unnecessary `buildscript.dependencies` and the implied 2nd spotless version definition.

build-logic/src/main/kotlin/polaris-java.gradle.kts

@@ -17,6 +17,8 @@
  * under the License.
  */
 
+import com.diffplug.spotless.FormatterFunc
+import java.io.Serializable
 import net.ltgt.gradle.errorprone.errorprone
 import org.gradle.api.tasks.compile.JavaCompile
 import org.gradle.api.tasks.testing.Test
@@ -80,19 +82,23 @@ tasks.withType(Jar::class).configureEach {
 }
 
 spotless {
-  val disallowWildcardImports = { text: String ->
-    val regex = "~/import .*\\.\\*;/".toRegex()
-    if (regex.matches(text)) {
-      throw GradleException("Wildcard imports disallowed - ${regex.findAll(text)}")
-    }
-    text
-  }
   java {
     target("src/main/java/**/*.java", "src/testFixtures/java/**/*.java", "src/test/java/**/*.java")
     googleJavaFormat()
     licenseHeaderFile(rootProject.file("codestyle/copyright-header-java.txt"))
     endWithNewline()
-    custom("disallowWildcardImports", disallowWildcardImports)
+    custom(
+      "disallowWildcardImports",
+      object : Serializable, FormatterFunc {
+        override fun apply(text: String): String {
+          val regex = "~/import .*\\.\\*;/".toRegex()
+          if (regex.matches(text)) {
+            throw GradleException("Wildcard imports disallowed - ${regex.findAll(text)}")
+          }
+          return text
+        }
+      },
+    )
     toggleOffOn()
   }
   kotlinGradle {

build.gradle.kts

@@ -20,12 +20,7 @@
 import java.net.URI
 import org.nosphere.apache.rat.RatTask
 
-buildscript {
-  repositories { maven { url = java.net.URI("https://plugins.gradle.org/m2/") } }
-  dependencies {
-    classpath("com.diffplug.spotless:spotless-plugin-gradle:${libs.plugins.spotless.get().version}")
-  }
-}
+buildscript { repositories { maven { url = java.net.URI("https://plugins.gradle.org/m2/") } } }
 
 plugins {
   id("idea")

gradle/libs.versions.toml

@@ -80,4 +80,3 @@ threeten-extra = { module = "org.threeten:threeten-extra", version = "1.8.0" }
 [plugins]
 openapi-generator = { id = "org.openapi.generator", version = "7.10.0" }
 rat = { id = "org.nosphere.apache.rat", version = "0.8.1" }
-spotless = { id = "com.diffplug.spotless", version = "6.25.0" }