Deprecate JDK8 images

[dduportal]

Jenkins requires Java 11 since June 2022 for the weekly release line and September 2022 for LTS release line.

For the jenkinsci/docker-agent and jenkinsci/docker-inbound-agent, a last release had been delivered a few month ago with the latest remoting agent.jar compatible with JDK8 (ref. jenkinsci/docker-agent#298).

For the SSH agent image here, there is no requirement around the agent.jar because it is not part of the image.
However it does not make sense to ship JDK8 version anymore.

Proposal: we should deprecate JDK8 SSH agent images (and associated tooling such as dependabot) in a new 5.x.y release line (current release is 4.10.0) on the principal branch.

The only "danger" is to be careful to properly write the changelog (either with good PR title and labelling to hint release-drafter, or even when publishing the release).

Ping @Poddingue @MarkEWaite @timja @NotMyFault @basil @halkeye @jglick @imonteroperez do you see any potential problem for this?

[timja]

no issues with it

[NotMyFault]

I'm fine with that proposal.

[dduportal]

• Implemented in breaking!: drop support for JDK8 #238
• Last release with JDK8: https://github.com/jenkinsci/docker-ssh-agent/releases/tag/4.16.0
• First release with only JDK11 and JDK17 for lInux (and only JDK11 for Windows): https://github.com/jenkinsci/docker-ssh-agent/releases/tag/5.0.0