Use ArrayList for storing parameters locally

Instead of using the Java 11-native `List.of` and `List.copyOf` to store
a copy of the parameters list locally, which aren't natively supported
by XStream serialization, use `ArrayList`, which is natively supported.
When returning the parameters, wrap them in an unmodifiable list to
avoid unexpected external modifications.

This unfortunately means that this class is not "immutable" anymore as
the parameters can technically be changed, but the current code does not
provide a way to accomplish that.

src/main/java/org/jenkinsci/plugins/scriptler/builder/ScriptlerBuilder.java

@@ -30,6 +30,7 @@
 import java.io.Serial;
 import java.io.Serializable;
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
@@ -108,7 +109,7 @@ public ScriptlerBuilder(
             @CheckForNull List<Parameter> parameters) {
         this.builderId = builderId;
         this.scriptId = scriptId;
-        this.parameters = parameters == null ? List.of() : List.copyOf(parameters);
+        this.parameters = new ArrayList<>(parameters == null ? List.of() : parameters);
         this.propagateParams = propagateParams;
     }
 
@@ -180,7 +181,7 @@ public Parameter[] getParameters() {
 
     @NonNull
     public List<Parameter> getParametersList() {
-        return parameters;
+        return Collections.unmodifiableList(parameters);
     }
 
     public String getBuilderId() {