stop dependabot trying to update GHATL

[mgravell]

Context #476, #479, #530

There is a hard dependency conflict here

What actually breaks (two layers):

1. GHATL 3.0 was rebuilt as a Microsoft.Testing.Platform (MTP) extension. Its MSBuild targets now inject a codegen hook (SelfRegisteredExtensions.cs) referencing the type GitHubActionsTestLogger.MtpIntegration. Our for the package omits compile (fine for 2.x, a pure VSTest logger with no compile-time types), so the generated code can't see the type → CS0400.
2. Add compile back and you hit the real wall:
   error CS1705: 'GitHubActionsTestLogger' 3.0.5 uses 'Microsoft.Testing.Platform 2.0.0'
   which has a higher version than referenced assembly 'Microsoft.Testing.Platform 1.9.1'

Why 1.9.1 is pinned: our xunit.v3 3.2.2 (current stable) drags in the xunit.v3.core.mtp-v1 / xunit.v3.mtp-v1 packages, which pin Microsoft.Testing.Platform to 1.9.1. GHATL 3.x is built against MTP 2.0. Those can't coexist. So the update genuinely cannot work today — it's blocked upstream on xunit, not on anything we're doing wrong.

When it will be possible: xunit is mid-migration from MTP v1 → v2. The mtp-v2 variant only exists in xunit.v3 4.0.0-pre.* (confirmed 4.0.0-pre.154 installs xunit.v3.mtp-v2). There is no stable xunit.v3 on MTP 2.0 yet, and Microsoft.NET.Test.Sdk latest stable is still 18.8.1 (what we have). So GHATL 3.x becomes viable only once we move to xunit.v3 4.x (MTP 2.0), which is still prerelease.

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Note

Low Risk
CI/dependency-bot configuration only; no runtime or test code changes.

Overview
Adds a Dependabot ignore for GitHubActionsTestLogger versions ≥ 3.0.0, so daily NuGet update PRs stop proposing upgrades that cannot build today.

The inline comment documents why: GHATL 3.x targets Microsoft.Testing.Platform 2.0, while the repo stays on xunit.v3 3.x and MTP 1.9.1 (via Directory.Packages.props at 2.4.1). Revisit when moving to xunit.v3 4.x / MTP v2.

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