Code generation documentation for Orleans should be updated for Orleans 7

[dggmez]

Issue description

Code generation documentation for Microsoft Orleans seems to be about Orleans 3.x and it looks like a lot of stuff works differently in Orleans 7 given that the linked packages and types do not exist anymore in Orleans 7:

https://learn.microsoft.com/en-us/dotnet/orleans/grains/code-generation

Target framework

• .NET 7

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Document Details

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• ID: 36be99c8-5b95-9b8e-1245-2626f7203085
• Version Independent ID: 0319fc47-717d-7cb1-3e44-09e711d3676a
• Content: Code generation
• Content Source: docs/orleans/grains/code-generation.md
• Product: dotnet-orleans
• GitHub Login: @IEvangelist
• Microsoft Alias: dapine

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Associated WorkItem - 91663