Restore throttling of jobs per node on Jenkins

[apupier]

seems that an upgrade of the jenkins plugin used changed the default value for maxConcurrentPerNode
jenkinsci/throttle-concurrent-builds-plugin@e761815 , so with this relase
https://github.com/jenkinsci/throttle-concurrent-builds-plugin/releases/tag/624.vc427fa_e0e503

When no value set previously, it was considering a value of one.

we are still not ready for playing the tests in //, there are still port conflicts and artifacts could overwrite them in the local repository causing wrong testing.

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[gnodet]

Straightforward fix — explicitly setting maxConcurrentPerNode: 1 after the throttle-concurrent-builds-plugin upgrade changed the default is the right call. The linked upstream commit confirms the parameter name and the behavior change.

One minor nit: there's a trailing blank line at line 44 in Jenkinsfile that isn't present in Jenkinsfile.jdk26. Not a blocker, but inconsistent.

Backports to 4.22.x (#26095) and 4.18.x (#26096) are already open. ✓

This review was generated by an automated reviewer (Hermès) on behalf of @gnodet.

[gnodet]

Nit: trailing blank line. Jenkinsfile.jdk26 doesn't have it — the two files should be consistent.

Suggested change

	throttleOption: 'category',
	maxConcurrentPerNode: 1