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

Now that the Preflight project was [renamed](https://jetstack.slack.com/archives/C0GAM2KV1/p1617103472097600) 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](https://github.com/jetstack/jetstack-secure-gcm/blob/main/README.md))
- The preflight agent → the Jetstack Secure control plane agent (in [README.md](https://github.com/jetstack/jetstack-secure-gcm/blob/main/README.md))
- Helm chart name "preflight" → "jetstack-secure-agent" (in the [jetstack-secure-gcm](https://github.com/jetstack/jetstack-secure-gcm/tree/main/chart/jetstack-secure-gcm) chart)
- Docker image "preflight-agent" → "jetstack-secure-agent"

What do you think @charlieegan3?

Review README.md and the chart/jetstack-secure-gcm directory, then search the repository for the existing “preflight” names. Update the specified dashboard, agent, Helm chart, and Docker image references consistently; done means the old names no longer appear in those locations.

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