chore: Align custom kotlinx-serializers to be objects

Emphasize that there never will be multiple instances of these.

Signed-off-by: Sebastian Schuberth <sebastian@doubleopen.org>

plugins/package-managers/pub/src/main/kotlin/Pubspec.kt

@@ -101,7 +101,7 @@ internal data class Pubspec(
  * If transformations like for JSON were available in kaml, this serializer could be simplified, see also
  * https://github.com/charleskorn/kaml/issues/29.
  */
-private class DependencyMapSerializer : KSerializer<Map<String, Dependency>> {
+private object DependencyMapSerializer : KSerializer<Map<String, Dependency>> {
     override val descriptor = serialDescriptor<Map<String, Dependency>>()
 
     override fun serialize(encoder: Encoder, value: Map<String, Dependency>) {

plugins/reporters/gitlab/src/main/kotlin/GitLabLicenseModel.kt

@@ -100,12 +100,12 @@ internal data class GitLabLicenseModel(
     )
 }
 
-private class SortedDependenciesListSerializer :
+private object SortedDependenciesListSerializer :
     KSerializer<List<GitLabLicenseModel.Dependency>> by sortedListSerializer(
         compareBy({ it.packageManager }, { it.name }, { it.version })
     )
 
-private class SortedLicenseSetSerializer :
+private object SortedLicenseSetSerializer :
     KSerializer<Set<GitLabLicenseModel.License>> by sortedSetSerializer(compareBy { it.id })
 
-private class SortedStringSetSerializer : KSerializer<Set<String>> by sortedSetSerializer(compareBy { it })
+private object SortedStringSetSerializer : KSerializer<Set<String>> by sortedSetSerializer(compareBy { it })