Serialization stability for `ParHashMap` is changing on Scala 3.3

[SethTisue]

as seen in recent CI runs e.g. https://github.com/scala/scala-parallel-collections/actions/runs/4139634855/jobs/7157433285

[error] Test scala.SerializationStabilityTest.testAll failed: java.lang.AssertionError: assertion failed: instance = ParMap(1 -> 2) : class scala.collection.parallel.immutable.ParHashMap
[427](https://github.com/scala/scala-parallel-collections/actions/runs/4139634855/jobs/7157433285#step:5:428)
[error]  serialization unstable: 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
[428](https://github.com/scala/scala-parallel-collections/actions/runs/4139634855/jobs/7157433285#step:5:429)
[error]    found: 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, took 0.059 sec
[429](https://github.com/scala/scala-parallel-collections/actions/runs/4139634855/jobs/7157433285#step:5:430)

the problem is also reproducible locally with ++3.3.0-RC2! followed by testOnly scala.SerializationStabilityTest

for now I think we can just let it keep failing, then once Scala 3.3 is out, we'll need a sequel to #223 for immutable.ParHashMap