terraform: Update kubeflow-dashboard module ci + docs

[orfeas-k]

Context

Update terraform module CI and docs according to agreed convention in canonical/training-operator#200 (comment) where we agreed that:

• Handling module compatibility in branches, that is, the module in Github branch track/1.8 of the training operator is compatible with that charm (1.8/stable, 1.8/edge), but the module in branch main is compatible with that charm (latest/edge).
• Removing the note from the README.md as it will be obviated that there is a 1:1 compatibility mapping between the module and the charm in the same Github branch

What needs to get done

• Remove the compatibility note from the README.md
• Update the terraform-checks.yaml call to:
  • omit model input, unless kubeflow needs to be defined. In that case, we should reference issue Centraldashboard does not find configmap if namespace is not "kubeflow" kubeflow/kubeflow#6136 with a comment-explanation.
  • omit channel input, since latest/edge is the new default one.
• Backport the merged PR to 1.9-related branch while updating the
  terraform-checks.yaml call to deploy track/stable charm.

Definition of Done

Module in main has been updated and backported to 1.9-related branch.

[syncronize-issues-to-jira]

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The internal ticket has been created: https://warthogs.atlassian.net/browse/KF-6317.

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[orfeas-k]

Closed by #237 and #238