Prepare 6.0.0 release

[oschwald]

Summary

• Bump MaxMind.GeoIP2 dependency from 6.0.0-beta1 to 6.0.0 (now GA on NuGet).
• Rename the dateless ## 6.0.0-beta1 release-notes heading to ## 6.0.0 (2026-05-22). Beta1 was never tagged or published, so collapsing it into the GA section avoids exposing an intermediate state no consumer experienced.
• Fold in four post-beta1 user-facing changes that hadn't been called out: required Tag on TransactionReport (BREAKING), ReportAsync argument validation, Guid.Empty → null normalization for MinFraudId, and a missing .ConfigureAwait(false) in ReportAsync.

<VersionPrefix> in the csproj is intentionally left at 6.0.0-beta1 — dev-bin/release.sh rewrites it automatically based on the topmost dated heading in releasenotes.md at release time.

Test plan

• dotnet build MaxMind.MinFraud.sln — 0 warnings, 0 errors across net8.0/net9.0/net10.0/netstandard2.0/netstandard2.1
• dotnet test --project MaxMind.MinFraud.UnitTest/MaxMind.MinFraud.UnitTest.csproj — 540/540 pass on net8.0, net9.0, net10.0
• precious tidy --all — clean (one minor wrap adjustment folded in)
• Verified release-notes coverage against `git log v5.3.1..HEAD` — all 9 user-facing library commits are described

🤖 Generated with Claude Code

[gemini-code-assist]

Code Review

This pull request updates the MaxMind.GeoIP2 dependency to version 6.0.0 and finalizes the release notes for the 6.0.0 release. Key changes include making the Tag property required in TransactionReport, implementing validation for required identifiers in ReportAsync, normalizing Guid.Empty values to null, and fixing a potential deadlock by adding ConfigureAwait(false). The reviewer suggested marking the new validation logic in ReportAsync as a breaking change in the release notes to ensure users are properly alerted to the new exception behavior.

[gemini-code-assist]

Since ReportAsync now throws an ArgumentException where it previously did not (when using object initializers), this change is technically a behavior change that can impact existing code. Consider marking this entry as BREAKING to alert users to the new validation logic, consistent with other breaking changes listed for this release.

Suggested change

	- `ReportAsync` on `MaxMind.MinFraud.WebServiceClient` now validates that the
	`TransactionReport` includes at least one of `IPAddress`, `MinFraudId`,
	`MaxMindId`, or `TransactionId`, throwing `ArgumentException` if none are set.
	Previously this check only ran in the (now obsolete) constructor.
	- **BREAKING:** `ReportAsync` on `MaxMind.MinFraud.WebServiceClient` now
	validates that the `TransactionReport` includes at least one of `IPAddress`,
	`MinFraudId`, `MaxMindId`, or `TransactionId`, throwing `ArgumentException` if
	none are set. Previously this check only ran in the (now obsolete)
	constructor.

[oschwald]

This may have made sense, but it is too late.