Now that the Preflight project was renamed to "Jetstack Secure control plane", we should rename all the mentions in the README to avoid confusion for the user.

Renames that I was thinking about:

• The Jetstack Secure dashboard → the Jetstack Secure control plane dashboard (in README.md)
• The preflight agent → the Jetstack Secure control plane agent (in README.md)
• Helm chart name "preflight" → "jetstack-secure-agent" (in the jetstack-secure-gcm chart)
• Docker image "preflight-agent" → "jetstack-secure-agent"

What do you think @charlieegan3?