Nodepool name - limit to 12 characters

[Gordonby]

PR Summary

Need to drop the limit from 20 to 12, to stop invalid nodepoolnames being used.

For Linux node pools, the length must be between 1 and 12 characters. For Windows node pools, the length must be between 1 and 6 characters. https://learn.microsoft.com/en-us/azure/aks/use-system-pools?tabs=azure-cli#limitations

PR Checklist

• PR has a meaningful title
• Summarized changes
• This PR is ready to merge and is not Work in Progress