Allow a single version of PromFed to support multiple Rancher Monitoring versions

[mallardduck]

Is your feature request related to a problem? Please describe.
Currently, PromFed only supports one Rancher Monitoring version because it only embeds one Rancher Project Monitoring Chart version. So that caused us to need to make a second layer of release branch to support 2 versions of Rancher Monitoring during periods of dual support.

However if we could bundle:
"the newest version of Rancher Project Monitoring for each active version of Rancher Monitoring on the target Rancher Minor version", then a single release branch can more easily support varying Rancher Monitoring versions.

Describe the solution you'd like
Step 1. Adjust build.yaml and geneate.go to allow multiple versions,
Step 2. Adjust PromFed helm chart to take input for what version of Project Monitoring to use
Step 3. When new Project Monitors are created use the new value (from step 2) to select the Project Monitoring Chart to use.

Note: Picking the RPM chart version to use should be flexible and well documented. For instance we can't rely on RPM version alone as that changes a lot; but maybe a map of Upstream version to RPM version. That way PromFed code can consistently map from 66.7 to RPM 0.5.x versions for instance.