Make the deprecation known to current users

[laushinka]

We will be deprecating definitely-gp, and we should notify users before doing so.

We can show this deprecation via:

• workspace
• the repo

[laushinka]

@jldec When do we want definitely-gp to be deprecated?

[jldec]

@laushinka the deprecation can start immediately, and if we do need to give exisiting users time to adapt their workflows, I think 6 weeks should be sufficient after the first warning.

[stale]

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