Bump Go version in go.mod to v1.13

Since we dropped support for v1.12 in
https://gitlab.com/gitlab-org/gitlab-workhorse/-/merge_requests/532, we
should set the go.mod version to the minimum supported version, which is
Go v1.13.

go.mod

 module gitlab.com/gitlab-org/gitlab-workhorse
 
-go 1.12
+go 1.13
 
 require (
 	github.com/BurntSushi/toml v0.3.1