Switch agent (java home) to JDK21 default

[dduportal]

Goal: switch all agent templates on each controller (where needed) with an implicit or explicit JDK default of 11/17 to 21.
This change could potentially break some build assuming a default java version implictly and requires #4124 to be finished.

Task list:

• Set JDK21 as default in the all-in-one agent template (packer-images). This impacts all Linux and Windows VM templates on ci.jenkins.io, trusted.ci.jenkins.io, cert.ci.jenkins.io and infra.ci.jenkins.io
• ci.jenkins.io
  • Linux container template jnlp
  • Linux container template jnlp-webbuilder
  • BOM Linux Container template
    • Set maven-bom to use JDK21 as default instead of JDK17
    • Propose a PR to https://github.com/jenkinsci/bom/blob/master/Jenkinsfile to simplify the node label (e.g. always use maven-bom)
• release.ci.jenkins.io
  • docker-packaging Linux
  • docker-packaging Windows
• (WiP)