Upgrade .NET SDK from RC 9.0.100-rc.2 to stable 9.0.306

[Copilot]

Upgrades from .NET 9 Release Candidate to the stable 9.0.306 release.

Changes

• global.json: SDK version 9.0.100-rc.2.24474.11 → 9.0.306, allowPrerelease disabled
• README.md: Updated installation instructions to reference stable SDK

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• api.cdnjs.com
  • Triggering command: /usr/share/dotnet/dotnet /usr/share/dotnet/sdk/9.0.306/MSBuild.dll /noautoresponse /nologo /nodemode:1 /nodeReuse:true /low:false (dns block)
• unpkg.com
  • Triggering command: /usr/share/dotnet/dotnet /usr/share/dotnet/sdk/9.0.306/MSBuild.dll /noautoresponse /nologo /nodemode:1 /nodeReuse:true /low:false (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

Upgrade the version of .net core .

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