Update New-AzNetworkInterface.md

[thomast1906]

Removed reference to the manual subnet entry and added code to allow retrieval of subnetID via PowerShell from Virtual Network.

This would be a best practice rather than hard-coding subnet ID entry.

Description

Checklist

• I have read the Submitting Changes section of CONTRIBUTING.md
• The title of the PR is clear and informative
• The appropriate ChangeLog.md file(s) has been updated:
  • For any service, the ChangeLog.md file can be found at src/{{SERVICE}}/{{SERVICE}}/ChangeLog.md
  • A snippet outlining the change(s) made in the PR should be written under the ## Upcoming Release header -- no new version header should be added
• The PR does not introduce breaking changes
• If applicable, the changes made in the PR have proper test coverage
• For public API changes to cmdlets:
  • a cmdlet design review was approved for the changes in this repository (Microsoft internal only)
  • the markdown help files have been regenerated using the commands listed here

[isra-fel]

@thomast1906 Thank you for the contribution 😀
@avijitgupta or @chandrasekarsrinivasan could you help review this doc update? Thanks.

[isra-fel]

LGTM

[isra-fel]

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