Document minimum required Helm version

In the file templates/observability-secret.yaml the following line

{{- if and ((.Values.config.observability).tls).enabled
           (((.Values.config.observability).tls).secret).create }}

causes this error [ERROR] templates/: parse error at (gitlab-agent/templates/observability-secret.yaml:1): unclosed action

Helm version: "v3.1.1"

Proposal

Document minimum required Helm version for this chart.