[SQL] Deprecate Get/Set LTR vault cmdlets

[adeal]

Description

Re-submission of previous PR to target preview branch instead of AzureRM.Sql.STR

This change deprecates two LTR cmdlets:

• Get-AzureRmSqlServerBackupLongTermRetentionVault
• Set-AzureRmSqlServerBackupLongTermRetentionVault

These cmdlets are no longer supported and a replacement is not needed.

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
  • the cmdlet markdown files were generated using the platyPS module

[maddieclayton]

Looks good! Thanks for updating.