Updates for .NET 8 release

[gewarren]

This issue tracks various updates needed for the .NET 8 release.

• Update the what's new article
• Update what's new in docs for .NET 8
• Update hub and landing pages with what's new article, etc.
• Remove "preview" references - including preview versions
• Refresh API docs and update moniker to remove "prerelease"
• Update API browser index
• Update .NET docs header (https://github.com/MicrosoftDocs/DocsRoot/pull/1752)
• Update zone pivots to include dotnet-8
• Update all 'updateeachrelease' articles
• Update ?view= links to latest version, as necessary
• Update support tables - e.g. https://docs.microsoft.com/en-us/dotnet/core/install/windows?tabs=net60#supported-releases
• Update .NET Standard support tables - https://learn.microsoft.com/en-us/dotnet/standard/net-standard
• Update the recommended target framework versions in the Open-source library guidance
• Update TFMs and preprocessor symbols - https://docs.microsoft.com/en-us/dotnet/standard/frameworks
• Remove preview-to-preview breaking changes
• Update list of APIs that always throw - https://docs.microsoft.com/en-us/dotnet/core/compatibility/unsupported-apis
• Update reference for settings that include a version:
  • AnalysisLevel and AnalysisMode
• Code analysis:
  • Add new code analysis rules:
    • CA1856
    • CA1857
    • CA2021
    • CA2261
    • CA1510-1513
    • CA1862
    • CA1863
  • Add new editorconfig settings (code style rules)
  • Update code analysis rules that are enabled by default - https://docs.microsoft.com/en-us/dotnet/fundamentals/code-analysis/overview#enabled-rules
  • Also update the info table in each CAXXXX rule article ("Enabled by default in .NET 8")
• Update SYSLIB warning documentation (obsoletions and source generators) - related to missing help for some source generator errors #35297
• Update version in breaking change issue template

Other considerations:

• Update samples and snippets to target new version (if it's LTS)
• Update code analysis level in samples (if necessary)
• Update tutorials to reflect latest version and any template/UI changes
• Update redirects that should always point to the latest version
• API docs: Unfilter any new attributes that should be shown in code signatures in the docs
• API docs: Update exceptions as necessary

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Associated WorkItem - 177279

[cremor]

A few additional points to consider:

• Update documentation for new code analysis error codes and new editorconfig settings.
• Update the recommended target framework versions in the Open-source library guidance