Update Jenkins core and plugins 2020 Q4

Upgrade:

• Jenkins to latest LTS currently available in their APT repository: 2.263.1
• Plugins (some for security needs, others just to run latest code)

Manually checked, for each plugin:

• Expected hashes (from https://updates.jenkins.io/download/plugins/).
• Latest versions and dependencies (from https://plugins.jenkins.io/).
• Changelogs for upgrades (from each package's GitHub page, when available).
• Compatibility with Jenkins version.