chore: bump MQTTnet from 4.2.1.781 to 4.3.1.873

[dependabot[bot]]

Bumps MQTTnet from 4.2.1.781 to 4.3.1.873.

Release notes

Sourced from MQTTnet's releases.

v4.3.1.873

• [Client] Fixed wrong TLS options handling (#1830).
• [Client] Fixed NullReferenceExeption when performing a Ping when the client is not connected (#1831).

v4.3.0.858

• [Client] Fixed PlatformNotSupportedException when using Blazor (#1755, thanks to @​Nickztar).
• [Client] Added hot reload of client certificates (#1781).
• [Client] Added several new option builders and aligned usage (#1781, BREAKING CHANGE!).
• [Client] Added support for RemoteCertificateValidationCallback for .NET 4.5.2, 4.6.1 and 4.8 (#1806, thanks to @​troky).
• [Client] Fixed wrong logging of obsolete feature when connection was not successful (#1801, thanks to @​ramonsmits).
• [Client] Fixed NullReferenceException when performing several actions when not connected (#1800, thanks to @​ramonsmits).
• [RpcClient] Added support for passing custom parameters to topic generation context (#1798, thanks to @​Temppus).
• [Server] Fixed NullReferenceException in retained messages management (#1762, thanks to @​logicaloud).
• [Server] Exposed new option which allows disabling packet fragmentation (#1753).
• [Server] Expired sessions will no longer be used when a client connects (#1756).
• [Server] Fixed an issue in connection handling for ASP.NET connections (#1819, thanks to @​CZEMacLeod).

Commits

• 6fdee9e Fix wrong TLS usage (#1833)
• cae7206 Fix NullReferenceException when calling Ping (#1834)
• e2e7aac Update version and release notes
• 0d3dd96 Samples that show concurrent processing (#1803)
• e0660ba #1810 Close connection after failed connect. (#1811)
• 5965ded Add possibility to specify parameter to RPC invocation (#1798)
• fc31c30 Fix Unit Tests
• e604345 Update version
• 111a0d6 Add support for hot reloading of client certificates (#1783)
• 1a08ec1 Added certificate validation callback for .NET framework 4.x (#1806)
• Additional commits viewable in compare view

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[dependabot[bot]]

Looks like MQTTnet is up-to-date now, so this is no longer needed.