[Feature]: Properly show warnings to the user through Diagnostics

[apricote]

What whould you like to see?

Right now when we want to warn the user of some upcoming changes and/or breaking changes we emit a warning log message. This is not surfaced to the user by default, they need to explicitly enable logs (TF_LOG=WARN) to see these messages.

• When the image used by a server is deprecated (source)
• (Soon) When the server type used by a server is deprecated (source)

The proper way to handle this is to send back a list of Diagnostics that each can be a warning or an error. These are properly rendered and shown by Terraform.

[github-actions]

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