Pin .NET SDK floor to 10.0.204 via global.json

[jschick04]

Why

SFI alert flagged that EventLogExpert ships a self-contained .NET application with an unpinned runtime version. The runtime baked into the published MSIX / EXE is whatever the active .NET SDK provides, and the repo had no global.json to enforce a secure minimum (the only build pipeline floated SDK via dotnet-version: 10.x). Alert recommendation: pin SDK floor to ≥ 10.0.204.

What

• Add global.json at repo root pinning SDK floor to 10.0.204 with rollForward: latestFeature. This enforces the SFI floor on every build (local dev, PR CI, downstream release pipeline) while allowing newer 10.0.x patches without manual bumps and without silently jumping to .NET 11.
• Update .github/workflows/PullRequest.yml to source the SDK from global.json (global-json-file: global.json) instead of the floating dotnet-version: 10.x.

The downstream Azure DevOps release pipeline will be updated in a separate PR to consume the same global.json.

Verification

• Locally: dotnet --info confirms global.json is detected at repo root and resolves to SDK 10.0.300 (satisfies the 10.0.204 floor via latestFeature).
• dotnet build EventLogExpert.slnx -c Release succeeds (41s, 0 warnings, 0 errors).
• All 2,175 unit tests pass: EventDbTool (8), Eventing (288), Filtering (869), Runtime (860), UI (150).