[SQL] Deprecate LTR retention vault cmdlets

[adeal]

Description

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]

@dealaus This looks great! Two comments though:

1. Can you target this at preview so that we get the deprecation in the next stable release?
2. Can you add back in the test, but add this around it to ensure that it only runs on Desktop?

#If !NETSTANDARD
        [Fact]
        [Trait(Category.AcceptanceType, Category.CheckIn)]
        public void TestServerBackupLongTermRetentionVault()
        {
            // TODO Rewrite SQL backup tests to be recordable
            // TODO https://github.com/Azure/azure-powershell/issues/4155
            if (TestMockSupport.RunningMocked)
            {
                RunPowerShellTest("Test-ServerBackupLongTermRetentionVault");
            }
        }
#Endif

[adeal]

I figured there was a why to not include the test for just netcore. Thanks!

Changing the base branch has quite the merge conflict. I think it'll be quicker to just abandon this and create a new PR targeting preview.