refactor(CopyrightGarbage): Avoid using a SortedSet

See #6235 for context. Here, the use of `StringSortedSetConverter` is
not required, as `ImportCopyrightGarbageCommand` is the only place that
serializes the class, and that uses its own `Collator` for custom
sorting.

Signed-off-by: Sebastian Schuberth <[email protected]>

Author: sschuberth

model/src/main/kotlin/config/CopyrightGarbage.kt

@@ -19,16 +19,14 @@
 
 package org.ossreviewtoolkit.model.config
 
-import java.util.SortedSet
-
 /**
  * A class that defines which Copyright statements are to be considered as garbage instead of real findings.
  */
 data class CopyrightGarbage(
     /**
      * A set of literal strings that identify garbage Copyright findings.
      */
-    val items: SortedSet<String> = sortedSetOf()
+    val items: Set<String> = emptySet()
 )
 
 fun CopyrightGarbage?.orEmpty(): CopyrightGarbage = this ?: CopyrightGarbage()