Update Jenkins core and plugins: 2020Q4 edition

From changelog and changelog-old, points of attention from 2.235.1 → 2.268 (current latest):

• 2.237: Update Apache Ant from 1.10.7 to 1.10.8. (pull 4725) -- We're running Ant 1.9.9-1+deb9u1.
• 2.240: Update bundled Script Security Plugin from 1.71 to 1.73. (pull 4769) -- We're using 1.74.
• 2.248: Remove the fallback Jenkins URL from the JNLP launch file so that WebSocket agents can be connected over Java Web Start. (issue 63014)
• 2.258 Update the bundled Mailer plugin from 1.21 to 1.32.1. (pull 4938) -- We're using 1.32.
• 2.259: Update the bundled version of Script Security Plugin from 1.73 to 1.75. (pull 4947) -- We're using 1.74
• 2.259: Update the bundled version of Display URL API plugin from 2.0 to 2.3.1. (pull 4948) -- We're using 2.3.2
• 2.260: Update bundled version of Apache Ant from 1.10.8 to 1.10.9. (pull 4958) -- We're running Ant 1.9.9-1+deb9u1.

Todo

• Update Jenkins
• Update plugins
• Restart all agents (to enforce download of fresh JNLP).
• Fix JNLP connection to use JENKINS_URL instead of always-working fallback URL.
• Cleanup Puppet code after deploy (remove disabled plugins).
• Remove disabled plugins.
• Revert jenkins.sib to production Puppet environment.
• Create issue for fixing APT proxy redirection problem -- #17797 (closed)
• Create issue for migrating away from JNLP, either using WebSockets or moving to controller → agent (SSH?) connection -- #17798 (closed).
• Create issue to puppetize and document the use of the fixed port 42585 for serving API: #17800 (closed)
• Check if there are plugins that were removed from Puppet but not from Jenkins.
• Upgrade to latest LTS (2.263.3)
• Schedule next update: #17799 (closed)

References

Last update: #16955 (closed)