Clarify messaging that the Operator is not OpenShift-only

Summary

At the moment, our GitLab Operator comes across as OpenShift-only. Some reasons for this include:

• The project path is gitlab-org/openshift/gitlab-operator
• Frequent references to OpenShift in our docs and scripts

Ultimately, we'd like the Operator to support both vanilla Kubernetes as well as OpenShift.

For the most part, the Operator in its current state should work on vanilla Kubernetes, but we have some dangling OpenShift-specific components in the codebase (especially in our startup scripts).

Action items

We should consider the following:

• Clearly communicate in the README.md that the project is expected to support the standard Kubernetes distribution as well as OpenShift
• Update scripts and the GitLab controller to only evaluate OpenShift-specific components if the target environment is OpenShift (i.e. OpenShift labels, OperatorHub references, etc.)