Bump to 6.28.0 and write the changelog

[jeremydmiller]

Release prep for 6.28.0.

• Directory.Build.props: 6.27.1 → 6.28.0
• CHANGELOG.md: entries for the six PRs in this wave, added at the top of ## Unreleased

Covered:

PR	What
#3931	OnMissing.EmptyContentWith204; [NoContentIfMissing] / [NotFoundIfMissing]; WolverineHttpOptions.OnMissingResponseBody
#3932	DateTime / DateTimeOffset now convention in Wolverine.HTTP
#3933	[FirstOrDefault] across Marten/Polecat/Fisher/RavenDb/EF Core
#3934	Storage.AppendEvents() / Storage.StartStream() event side effects
#3936	[All] + [Queryable], and the CanApply event-operations transaction fix
#3938	[All] query batching on Marten/Polecat/Fisher
#3939	GH-3937: provider errors name the declaring method

#3938 also carried a test-infrastructure fix (config file watchers leaking in test hosts) that ships in no package, so it has no changelog entry of its own.

Must merge before publish_nugets.yml runs — that workflow checks out main with no ref.

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