Prepare Helmfile 1.0 upgrade

Helmfile will soon be released in 1.0 version, at this moment the semver rules will be enforced on the project.

The helmfile team has some breaking change for this release: https://github.com/helmfile/helmfile/blob/main/docs/proposals/towards-1.0.md

We are impacted by two points:

• Forbid the use of environments and releases within a single helmfile.yaml.gotmpl part
• Force .gotmpl file extension for helmfile.yaml in case you want helmfile to render it as a go template before yaml parsing.

The first one shouldn't be an issue.
The second one will need an update in the shared library to test which helmfile extension to take.