update go.mod

[udf2457]

Please bring the modules and go.mod itself up to date.

For example, go.mod itself references go 1.15. This needs to be kept inline with Go release policy of "current and two prior". So at this late stage you should really bump up to 1.21, because 1.20 will be EOL within a few months.

The version of Go referenced is not cosmetic. Keeping it up to date has many benefits as outlined here

Keeping modules themselves up to date is of course important for obivous reasons, for example, your NATS references 1.15 and NATS is now at 1.34 !