sourcehut: update.sh: bump `gqlgenVersion` on update too
This extends the current script to pull the gqlgen version from the
go.mod file in the repository root (which we check out already anyway)
and replace it as necessary.
Makes updating all packages _a lot_ less painful. And since this is now
automated, remove the default value we previously defined for
`gqlgenVersion`.
Signed-off-by: Christoph Heiss <christoph@c8h4.io>