Align the wording on `dotnet workload config --update-mode` to not mention 'install mode'

[baronfel]

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>dotnet workload config --update-mode workload-set
Successfully updated workload install mode to use workload-set.

The message here mentions 'install mode', but the command looks like 'update mode' - this is ambiguous and unclear to users. We should mirror the wording used in 'update mode' in the message, even though we are talking about "updating the mode" here and not the "update mode"