Switch go.mod version to Go 1.21.0

[mx-psi]

On a38bb01 the go directive was changed to go 1.21.9. Quoting the Go mod file reference:

A go directive indicates that a module was written assuming the semantics of a given version of Go.

I believe this can be safely changed to go 1.21.0, since the semantics are similar between Go 1.21.0 and Go 1.21.9. The current version makes it impossible for modules that depend on go-hdb to be built with Go versions between 1.21.0 and 1.21.9.

[stfnmllr]

@mx-psi yes, thanks - go directive is change to 1.21.0 with go-hdb v1.8.20

[mx-psi]

Thank you for the extremely quick turn around! :)