Filter opaque modifier from object documentation (#21640)

Fixes #21228

Author: mbovel

scaladoc-testcases/src/tests/opaqueTypes.scala

@@ -6,4 +6,8 @@ opaque type Permissions
  = Int
 opaque type PermissionChoice
  = Int
-//opaque type Permission <: Permissions & PermissionChoice = Int TODO: #112
+//opaque type Permission <: Permissions & PermissionChoice = Int TODO: #112
+
+object Foo:
+  opaque type Bar
+   = Int

scaladoc/src/dotty/tools/scaladoc/tasty/ClassLikeSupport.scala

@@ -314,7 +314,7 @@ trait ClassLikeSupport:
   def parseObject(classDef: ClassDef, signatureOnly: Boolean = false): Member =
     mkClass(classDef)(
       // All objects are final so we do not need final modifier!
-      modifiers = classDef.symbol.getExtraModifiers().filter(_ != Modifier.Final),
+      modifiers = classDef.symbol.getExtraModifiers().filter(mod => mod != Modifier.Final && mod != Modifier.Opaque),
       signatureOnly = signatureOnly
     )