Deprecate DisableSoftDelete in New-AzKeyVault & EnableSoftDelete in Update-AzKeyVault

[BethanyZhou]

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]

I have considered that we don't have to keep -EnableSoftDelete in the update cmdlet.
We also don't have to add notes in help docs, because user can get the info in changelog.
So I'll just merge the PR.
Thanks