Add global.json for .NET 8 SDK; replace explicit versions in CI workflows

[Copilot]

Centralizes the .NET 8 SDK version into a single global.json rather than repeating it across every workflow file.

Changes

• global.json (new, repo root): pins .NET 8 SDK at 8.0.100 with rollForward: latestFeature, equivalent to the previous 8.0.x wildcard
• build.yml: both setup-dotnet steps now use global-json-file: global.json instead of dotnet-version: '8.0.x'
• publish.yml: same replacement
• test-documentation.yml: same replacement
• test-code.yml: removes 8.0.x from the multi-SDK dotnet-version list; adds global-json-file: global.json as a sibling parameter — actions/setup-dotnet installs all versions from both inputs

# test-code.yml — before
dotnet-version: |
  2.1.x
  3.1.x
  6.0.x
  8.0.x

# after
dotnet-version: |
  2.1.x
  3.1.x
  6.0.x
global-json-file: global.json