Adapt some implicit resolution change in 3.5

There is a conflict between opaqueHandler and userIdOfWriter for UserId
Both are sastify for BSONWriter[UserId]. In this case of conflicting,
before 3.5 the compiler will pick the more specific type (in this case
opaqueHandler: BSONHandler[UserId] because it's more specific than
BSONWriter[UserId]). In 3.6 they will flip the priority, a.k.a pic the
most general. So, this specific case doesn't really affect us. But
removing implicit ambiguity is good anyway I guess.

More detail on the implicit resolution change here:
scala/scala3#19300

modules/db/src/main/Handlers.scala

@@ -33,6 +33,9 @@ trait Handlers:
   given userIdOfWriter[U: UserIdOf, T](using writer: BSONWriter[UserId]): BSONWriter[U] with
     inline def writeTry(u: U) = writer.writeTry(u.id)
 
+  given NoDbHandler[UserId] with {}
+  given userIdHandler: BSONHandler[UserId] = stringIsoHandler
+
   given dateTimeHandler: BSONHandler[LocalDateTime] = quickHandler[LocalDateTime](
     { case v: BSONDateTime => millisToDateTime(v.value) },
     v => BSONDateTime(v.toMillis)