Serialization stability failure on Scala 3.next

[SethTisue]

https://github.com/scala/scala-parallel-collections/runs/7287860376

[error] Caused by: java.io.InvalidClassException: scala.collection.immutable.OldHashSet$HashTrieSet; local class incompatible: stream classdesc serialVersionUID = -2139837478000879198, local class serialVersionUID = 4909220184304460089

last green run was https://github.com/scala/scala-parallel-collections/runs/6976037468 on 3.1.3-RC5
first red run was https://github.com/scala/scala-parallel-collections/runs/6995343025 on 3.2.0-RC1

reproducible locally with ++3.2.0-RC2; testOnly *Stability*

probably relevant history: #220 (0409c4f), scala/scala-dev#809

[SethTisue]

PR validation doesn't check this; but post-merge, there was a green run at https://github.com/scala/scala-parallel-collections/runs/7291145624 👍