[Elastic Agent] Document how to upgrade an fleet managed agent running on containers

[AndersonQ]

Currently our documentation does not explain how to upgrade those agents. Actually it does not explain those agents cannot be upgraded through fleetUI and it should be done by changing the agent's image in the k8s manifest file, or where ever the docker image is defined.

[cmacknz]

This doesn't just affect k8s, but any situation where the agent is running in a container. To upgrade you need to change the container image, which Fleet cannot do.

Fleet could tell the agent process in the container to upgrade, and it would work temporarily, but the agent would revert to the previous version the next time the container restarts. The agent version installed and launched in the container at startup is fixed once the container is built and cannot be changed at runtime.