Set GKE release_channel to unspecified to prevent auto k8s updates

[iameskild]

Reference Issues or PRs

What does this implement/fix?

Put a x in the boxes that apply

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds a feature)
• Breaking change (fix or feature that would cause existing features not to work as expected)
• Documentation Update
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Build related changes
• Other (please describe):

Testing

• Did you test the pull request locally?
• Did you add new tests?

I created a new Nebari cluster to test that this would work as expected. I also upgraded an existing cluster to validate that it would work for those clusters as well. No issues detected. Node pools may be destroyed and recreated if there are version differences. More information about release_channel can be found here and here.

Any other comments?

[pavithraes]

Thanks!