`MySqlConnection.DisposeAsync` doesn't raise `Disposed`

[bgrainger]

MySqlConnection.Dispose raises the Disposed event (via Component.Dispose). MySqlConnection.DisposeAsync doesn't.

It probably should, for clients that depend on the Disposed event. See PomeloFoundation/Pomelo.EntityFrameworkCore.MySql#1718.

[bgrainger]

Fixed in 2.2.0.