🔙 Backport 2c15276 from v5/master to master

- Compared to 2c15276: changed socketsHttpHandler to HttpClientHandler due to legacy .NET Framework
- Use System.Net.Http.HttpClient for GeositeUpdater
- New update check mechanism: first download checksum and compare, only download GeoSite DB on different checksum
- Verifiy downloaded GeoSite DB by comparing sha256sum before committing the change