Switch to defradb/errors.Join from stdlib/errors.Join

[fredcarle]

Some devs have started using Join from the std lib errors package. This will eventually cause us to import both the std lib errors and Defra's errors packages in the same file. Lets add Join to Defra errors to avoid this.

Edit: It's already part of Defra's errors package so we should switch out where we use stdlib/errors.Join for defradb/errors.Join.