Update dotnet monorepo to v9 (major)

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
mcr.microsoft.com/dotnet/aspnet	stage	major	8.0-alpine -> 9.0-alpine
mcr.microsoft.com/dotnet/sdk	final	major	8.0 -> 9.0
mcr.microsoft.com/dotnet/sdk	stage	major	8.0 -> 9.0

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