Stop relying on deprecated `/computer/${NAME}/jenkins-agent.jnlp` endpoint

[shamil]

Since Jenkins 2.437 (and the LTS 2.440.1+), authenticating the agent using Jenkins auth is deprecated, a node secret should be used instead. The solution I've in mind is to switch to Jenkins swarm plugin which handles this autmatically.

References:

• jenkinsci/jenkins#8773
• jenkinsci/swarm-plugin#601