Adjust Jenkins configuration to allow more memory per JVM (fixes #20819)

[kennknowles]

I've seen a few more of "warning: Memory constraints are impeding performance; please increase max heap size.". This combines with -Werror to unfortunately fail the build. We haven't seen these in a while because we increased the heap a decent amount. But still our heaps are tiny by modern standards.

This PR adjusts the balance between memory and Gradle processes to give a bigger heap to fewer JVMs. We can see how it impacts performance of the builds.

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