Fix Javadoc warnings due to Java 21 update (+ some other warnings)

The default implicit constructor must also have a doc comment, so I have to make it explicit and either properly restrict the visibility of the constructor, or actually document it.

pandalib-core/src/main/java/fr/pandacube/lib/core/backup/BackupCleaner.java

@@ -70,6 +70,11 @@ public abstract class BackupCleaner implements UnaryOperator<TreeSet<LocalDateTi
         };
     }
 
+    /**
+     * Create a backup cleaner.
+     */
+    public BackupCleaner() {}
+
 
     /**
      * Creates a new {@link BackupCleaner} that keeps the archives kept by this {@link BackupCleaner} or by the provided

pandalib-core/src/main/java/fr/pandacube/lib/core/cron/CronScheduler.java

@@ -54,7 +54,7 @@ public class CronScheduler {
                 long now = System.currentTimeMillis();
 
                 if (!tasks.isEmpty()) {
-                    CronTask next = tasks.get(0);
+                    CronTask next = tasks.getFirst();
                     if (next.nextRun <= now) {
                         next.runAsync();
                         setLastRun(next.taskId, now);
@@ -224,5 +224,6 @@ public class CronScheduler {
                 .toEpochMilli();
     }
 
+    private CronScheduler() {}
 
 }

pandalib-core/src/main/java/fr/pandacube/lib/core/json/Json.java

@@ -163,4 +163,6 @@ public class Json {
 		}
 	}*/
 
+	private Json() {}
+
 }

pandalib-core/src/main/java/fr/pandacube/lib/core/json/ThrowableAdapter.java

@@ -30,6 +30,8 @@ public class ThrowableAdapter implements JsonSerializer<Throwable>, JsonDeserial
 
     /* package */ static final TypeAdapterFactory FACTORY = TreeTypeAdapter.newTypeHierarchyFactory(Throwable.class, new ThrowableAdapter());
 
+    private ThrowableAdapter() {}
+
 
     @Override
     public Throwable deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException {

pandalib-core/src/main/java/fr/pandacube/lib/core/json/TypeConverter.java

@@ -237,5 +237,8 @@ public class TypeConverter {
 		}
 		
 	}
+
+
+	private TypeConverter() {}
 	
 }

pandalib-core/src/main/java/fr/pandacube/lib/core/mc_version/MinecraftVersionList.java

@@ -58,6 +58,9 @@ public record MinecraftVersionList(
 
         private static final TypeToken<Map<String, Integer>> MAP_STR_INT_TYPE = new TypeToken<>() { };
         private static final TypeToken<Map<Integer, List<String>>> MAP_INT_LIST_STRING_TYPE = new TypeToken<>() { };
+
+        private MinecraftVersionListAdapter() {}
+
         @Override
         public MinecraftVersionList deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException {
             if (!(json instanceof JsonObject jsonObj))

pandalib-core/src/main/java/fr/pandacube/lib/core/mc_version/MinecraftVersionUtil.java

@@ -135,4 +135,7 @@ public class MinecraftVersionUtil {
         return set;
     }
 
+
+    private MinecraftVersionUtil() {}
+
 }