Migrate Easify.Notifications to .Net 6.0 (#6) * [DEV-1929] Upgraded to .net 6 * [DEV-1929] Namespace and global using updates * [DEV-1929] Updated ci.yaml * [DEV-1929] Added global system usings file * [DEV-1929] Update to release.yaml file * [DEV-1929] Upgraded all NetStandard projects to Net 6 * [DEV-1929] Small usings file tidy up