Description
When using async sinks with multiple sinks dispose does not work.
The async sink tries to dispose its childs but the child is a SafeAggregateSink that is not disposable and therefore breaking the cleanup. as SafeAggregateSink is part of the core I opened the bug here.

Reproduction

var logger = new Configuration().WriteTo.Async(a => a.WriteTo.File("test.log").WriteTo.Console()).CreateLogger();
logger.Dispose();

 logger = new Configuration().WriteTo.Async(a => a.WriteTo.File("test.log")).CreateLogger();  // throws file in use exception

Expected behavior
the first logger should be disposed correctly

Relevant package, tooling and runtime versions
serilog.core