Removing usage of https://github.com/hashicorp/go-multierror

[elordis]

@roblaszczak, @m110
Hello, team.
Our security guys recently noticed that Watermill uses https://github.com/hashicorp/go-multierror package, which uses MPL-2.0 License, which is a bit of a pain for us. After a quick look at the watermill code I've noticed that it would be trivial to replace go-multierror usage with standard library implementation. The only downside that I see is that errors.Join appeared in Go 1.20, but current Watermill is already at 1.21 and I didn't notice any version compatibility guarantees in documentation.

Are there any plans for such a change? Are you willing to accept a Pull request if we do it ourselves?