Fix: Use the extension type instead of the name when disabling vmss disk encryption

[danwkennedy]

Description

Fixes #5038 . Since the extension name attribute can be set by the user, look for the extension by its type in the list of VMSS extensions.

Checklist

• I have read the Submitting Changes section of CONTRIBUTING.md
• The title of the PR is clear and informative
• The appropriate change log has been updated
• 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:
  • the changes have gone through a cmdlet design review (Microsoft internal partners only)
  • the cmdlet markdown files were generated using the platyPS module

[adxsdkps]

Can one of the admins verify this patch?

[msftclas]

All CLA requirements met.

[cormacpayne]

@danwkennedy Hey Daniel, thanks for the contribution. Before we can do anything with this PR, we'll need you to sign the CLA. Please see the comment above by the @msftclas bot for more information.

[danwkennedy]

@cormacpayne Linked my MS and GH accounts so the CLA should be good to go.

[cormacpayne]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 3 pipeline(s).

[cormacpayne]

@hyonholee @ejarvi would you mind signing-off on the changes in this PR?

[ejarvi]

Looks good to me - let me know if anything else other than this confirmation needed for sign off.