Rename all occurences of "preflight" with "Jetstack Secure platform control plane"

[maelvls]

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?

[maelvls]

Note that the "initial" public release of the Marketplace application has not been done yet, so we can still change the Helm chart names easily. Later on, it will become harder.

[charlieegan3]

Thanks Mael, we need to update the preflight agent image repo name. I've added that to the list for next sprint our side.