[SRP] support new API version 2021-01-01, and upgrade version to 19.0.0

[blueww]

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  [wei] [Microsoft.Storage] Add Jan21 api-version. azure-rest-api-specs#12807
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  [wei] this is on a new API version, and on a new major SDK version, the breaking change are added in change log.

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[blueww]

The only CI failure is not valid:

The failure is on verify generated code, error is "Discrepancy detected between generated code in PR and reference generation." But I have not change the code after generate it.

Since autorest issue Azure/autorest.csharp#885, (.net SDK code should skip validate optional properties if it's null), generate .net SRP SDK code must use --use:@microsoft.azure/autorest.csharp@2.3.90 now, and I have added it into generate.ps1.

But the code generated by CI will always check the optional properties no matter it's null or not, so the CI generated code seems not right. It seems CI don't generate SDK code use the correct autorest version, so it might not use generate.ps1 correctly.