Adjust index generation to support multiple RCs

[MbolotSuse]

Overview

Right now, make charts does not support adding multiple RCs of the same version. For example, if both webhook version 2.0.5+up0.3.5-rc5 and 2.0.5+up0.3.5-rc6 are in the release.yaml and have assets/charts generated, the index will claim to be up to date, even though rc6 is not included in the index.yaml. This will make it so that helm clients are unable to discover the rc6 version.

Background

As part of rancher/rancher#40159 we are trying to pin exact versions of the webhook, in order to provide more reliable CI pipelines and more clearly meet the requirement of the webhook being 1-1 with a given rancher version. This change is necessary to make it so that new CI pipelines in rancher/rancher can use the newer versions of the webhook, but other PRs which don't change this verison can continue to use older rc versions of the webhook.