Including option to use VMSS Flex

[danycontre]

Overview/Summary

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This PR fixes/adds/changes/removes

1. Including portal UI and Bicep/ARM option to enable (default) or not the use of VMSS Flex.

Breaking Changes

1. New parameter (deployVmssFlex) added with default value of true

Testing Evidence

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As part of this Pull Request I have

• Read the Contribution Guide and ensured this PR is compliant with the guide
• Ensured the resource API versions in .bicep file/s I am adding/editing are using the latest API version possible
• Checked for duplicate Pull Requests
• Associated it with relevant GitHub Issues
• (AVD LZA Team Only) Associated it with relevant ADO Items
• Ensured my code/branch is up-to-date with the latest changes in the main branch
• Performed testing and provided evidence.
• Updated relevant and associated documentation (e.g. Contribution Guide, Module READMEs, Docs etc.)