`azurerm_kubernetes_cluster` - set `host_encryption_enabled` and remove 4.0 flags

[catriona-m]

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Description

This PR fixes a bug where host_encryption_enabled is not being set correctly and removes 4.0 flags around these since 4.0 has now been released

PR Checklist

• [x] I have followed the guidelines in our Contributing Documentation.
• [x] I have checked to ensure there aren't other open Pull Requests for the same update/change.
• [x] I have checked if my changes close any open issues. If so please include appropriate closing keywords below.
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Testing

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Testing in TC

Change Log

Below please provide what should go into the changelog (if anything) conforming to the Changelog Format documented here.

• azurerm_kubernetes_cluster - host_encryption_enabled and node_public_ip_enabled are now set correctly

This is a (please select all that apply):

• [x] Bug Fix
• [ ] New Feature (ie adding a service, resource, or data source)
• [ ] Enhancement
• [ ] Breaking Change

Related Issue(s)

Fixes #27201

[!NOTE] If this PR changes meaningfully during the course of review please update the title and description as required.