[App] Remove --instance-types from cluster creation

[luca3rd]

What does this PR do?

Removes the ability to specify --instance-types when creating clusters.

Instead, all clusters will be able to use every instance type supported by the platform.

NOTE

Users can still set custom instance types with the --edit-before-creation option. Since this is considered an "advanced" option (you're modifying the JSON just before it's sent to the platform), we will allow this behaviour.

Does your PR introduce any breaking changes? If yes, please list them.

Yes:

• lightning create cluster ... --instance-types will no longer work.

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cc @Borda