Add Java 21 upgrade instructions

[kmartens27]

This PR is to add instructions on upgrading to Java 21. It includes the video that Darin Pope has recorded, and step by step instructions on how to perform the upgrade for both the controller and agents. It also adjusts some of the section headings to adjust -ing verbs and phrases.

[kmartens27]

preview deploy link: https://deploy-preview-8046--jenkins-io-site-pr.netlify.app/doc/book/platform-information/upgrade-java-to-21/

[kcsmalls]

LGTM. Left a few comments.

[gounthar]

Fantastic work, Kevin! 🚀

[kmartens27]

Thanks @gounthar, I forgot to call out that the installation instructions may change depending on the package manager.

[gounthar]

That was nitpicking from me...

[kmartens27]

@gounthar, I know you've already reviewed, but from your experience/what you have on your setup, is the updated Linux installation information accurate as well? I checked it out on my mac but some parts of it will be different since it's providing Debian instructions.

[gounthar]

Yes, @kmartens27 , it did work for my Debian machines. 👍

sudo apt install  temurin-21-jdk
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
[...]
The following NEW packages will be installed:
  adoptium-ca-certificates fonts-dejavu-extra p11-kit p11-kit-modules temurin-21-jdk
0 upgraded, 5 newly installed, 0 to remove and 4 not upgraded.
[...]

sudo update-alternatives --config java
There are 2 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                         Priority   Status
------------------------------------------------------------
* 0            /usr/lib/jvm/java-21-openjdk-arm64/bin/java   2111      auto mode
  1            /usr/lib/jvm/java-21-openjdk-arm64/bin/java   2111      manual mode
  2            /usr/lib/jvm/temurin-21-jdk-arm64/bin/java    2111      manual mode

Press <enter> to keep the current choice[*], or type selection number: 2
update-alternatives: using /usr/lib/jvm/temurin-21-jdk-arm64/bin/java to provide /usr/bin/java (java) in manual mode

[kmartens27]

Thanks very much for the confirmation @gounthar, I appreciate it greatly.

[gounthar]

You're welcome, Kevin! 😊