.NET 10 update

[et1975]

• Bumping Major to 3
• Update to target .NET 10
• Pin the solution dependencies via CPM

[cataggar]

Overall this is a clean, well-structured .NET 10 upgrade. CI is green. Approving with two minor notes:

What looks good:

• Major version bump to 3.0.0 is appropriate for the TFM breaking change
• Central Package Management (CPM) with CentralPackageTransitivePinningEnabled is the right approach for reproducible builds
• global.json with rollForward: latestPatch is a good balance between pinning and flexibility
• CI workflow correctly uses global-json-file instead of a hardcoded dotnet-version — single source of truth
• All dependency versions are explicitly pinned in Directory.Packages.props

Minor nits (non-blocking):

1. PR description vs implementation: The description mentions "solution lock file" but the actual mechanism is CPM with transitive pinning — not a NuGet lock file (packages.lock.json / RestorePackagesWithLockFile). The CPM approach is arguably better, but the description could be clarified.

2. OpenTelemetry.Extensions.Hosting at 1.0.0-rc9.5: This pre-release version is pre-existing, but worth checking if a stable release is available now given all the other deps were bumped.