Utilise the migrate template API when applying changes to the deployment template id

[tobio]

Description

Use the migrate template API to build the base update request when an existing deployment changes. deployment_template_id,

Related Issues

Closed https://github.com/elastic/terraform-provider-ec/issues/487

Motivation and Context

This API is purpose built to update deployment options to be compatible with a different template and simplifies the process for migrating deployments between hardware types.

How Has This Been Tested?

Manually, acceptance test, unit tests

Types of Changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Refactoring (improves code quality but has no user-facing effect)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Documentation

Readiness Checklist

• [x] My code follows the code style of this project
• [ ] My change requires a change to the documentation
• [ ] I have updated the documentation accordingly
• [x] I have added tests to cover my changes
• [x] All new and existing tests passed