After configuring Blob in Azure Function (.NET framework isolated v4), an error message "Feature 'using declarations' is not available in C# 7.3. Please use language version 8.0 or greater" appeared.

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REPRO STEPS

1. File > New > Project > Azure Functions > .NET Framework lsolated v4 ~ select "Blob trigger" and set the connection string setting name with "AzuriteStorage" > Create.
2. Select "Storage Azurite emulator (Local)" > Next, Keep the connection string name and value default> Next > Finish.

Internal bug: https://dev.azure.com/devdiv/DevDiv/_workitems/edit/1874704/

Introduced here:
daf6b58

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This is happening because net48 uses LangVersion 7.3 by default.

Possible fixes:
Update just net48 project template to use LangVersion 8.0 explictly.
Update all project templates to use 'latest'
Update project templates to pick a particular known good langversion.

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@mattchenderson @fabiocav FYI, this is an older issue, but I've been going through the internal backlog and am going to resolve the internal issue as external to this issue.