Update OpenTelemetry to 1.15.2

[martincostello]

Changes

• Update OpenTelemetry dependencies for 1.15.2.
• Update test NuGet packages to their latest versions.

Merge requirement checklist

• Unit tests added/updated
• CHANGELOG.md updated
• Changes in public API reviewed (if applicable)

[Copilot]

Pull request overview

Updates dependency versions to pick up OpenTelemetry .NET 1.15.2 (plus a few related package bumps), and records the change in the changelog.

Changes:

• Bump core OpenTelemetry packages (OpenTelemetry, OpenTelemetry.Exporter.OpenTelemetryProtocol, OpenTelemetry.Extensions.Hosting) to 1.15.2.
• Update test infrastructure package Microsoft.NET.Test.Sdk to 18.4.0.
• Refresh example project dependencies (including OpenTelemetry.Exporter.Console 1.15.2 and AWSSDK.S3).

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/Grafana.OpenTelemetry.Tests/Grafana.OpenTelemetry.Tests.csproj	Updates the test SDK package version.
src/Grafana.OpenTelemetry/Grafana.OpenTelemetry.csproj	Updates OpenTelemetry.Extensions.Hosting to 1.15.2.
src/Grafana.OpenTelemetry.Base/Grafana.OpenTelemetry.Base.csproj	Updates core OpenTelemetry + OTLP exporter to 1.15.2.
examples/net10.0/aspnetcore/aspnetcore.csproj	Updates example dependencies including OpenTelemetry console exporter.
CHANGELOG.md	Adds release notes for 1.6.1 reflecting the OpenTelemetry 1.15.2 updates.