support/errors: mark and document that support/errors is deprecated

[leighmcculloch]

Given @Shaptic's comments below we should mark and document that support/errors is deprecated so that folks avoid writing new code using it.

We've been trying to move away from this package which is basically an ancient ancestor that was written prior to Go's native errors package being actually decent. Instead of Wrapf we've been using errors.Join, fmt.Errorf, etc.

Originally posted by @Shaptic in https://github.com/stellar/go/pull/5708#discussion_r2104998019