SymSpell v6.7.3

• TargetFrameworks changed from netstandard2.0;net461;net47;netcoreapp3.0 to netstandard2.0;net9.0.
• PackageReferences updated.
• In SymSpell.Test all Assert.AreEqual changed to Assert.That
• Incorporates PR #126 that fixes null reference exception in CommitStaged (#139).