Gracefully allow upgrade of versions before observability config

The config.observability config didn't always exist and when upgrading from an older version where it didn't exist and the values are re-used it would result in a templating error like this:

Error: UPGRADE FAILED: template: gitlab-agent/templates/deployment.yaml:32:12: executing "gitlab-agent/templates/deployment.yaml" at <include "gitlab-agent.annotations" $podAnnotations>: error calling include: template: gitlab-agent/templates/_helpers.tpl:72:23: executing "gitlab-agent.annotations" at <$.Values.config.observability.enabled>: nil pointer evaluating interface {}.enabled