Updated Set-AzExpressRouteGateway to allow either one of -MinScaleUnits and -MaxScaleUnits

[nimaller]

Description

Currently, when using set-azexpressroutegateway, users can specify min scale units or max scale units, and the request will succeed but the values will be ignored. This PR fixes it by validating the range in min and max scale units, and allowing only one of them to be specified.

Copied from: #14145

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
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• 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:
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  • the markdown help files have been regenerated using the commands listed here

[VeryEarly]

Hi @nimaller ,

Please suppress the breaking change message into
https://github.com/nimaller/azure-powershell/blob/nimaller/setExpressRouteGatewayFix/tools/StaticAnalysis/Exceptions/Az.Network/BreakingChangeIssues.csv