RFR: Use only up to ConcGCThreads for concurrent RS scanning.

[William Kemper]

On Sat, 27 Aug 2022 01:16:55 GMT, Y. Srinivas Ramakrishna <ysr at openjdk.org> wrote:

> The concurrent RS scanning phase was using (up to) ParallelGCThreads, rather than ConcGCThreads as was the original intention for concurrent phases.

Marked as reviewed by wkemper (Committer).

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PR: https://git.openjdk.org/shenandoah/pull/159