Update minimum supported Go version 1.15 -> 1.17

The latest version of some packages we depend on (golang.org/x/sys,
indirectly) do not support 1.15 anymore.

We kept 1.15 because it's the version on Debian stable, but that release
is more than 2y old, and newer versions are readily available in Debian
stable backports.

So this patch updates the minimum supported Go version to 1.17.

.gitlab-ci.yml

@@ -16,9 +16,9 @@ stages:
     - su testing -c "go test ./..."
     - su testing -c "go test -race ./..."
 
-golang_1.15:
+golang_1.17:
   <<: *golang
-  image: golang:1.15    # Version in Debian stable.
+  image: golang:1.17    # Oldest supported Go version.
 
 golang_latest:
   <<: *golang

go.mod

@@ -1,6 +1,6 @@
 module blitiri.com.ar/go/chasquid
 
-go 1.15
+go 1.17
 
 require (
 	blitiri.com.ar/go/log v1.1.0
@@ -12,6 +12,10 @@ require (
 	golang.org/x/term v0.2.0
 	golang.org/x/text v0.4.0
 	golang.org/x/tools v0.1.12
-	golang.org/x/xerrors v0.0.0-20200804184101-5ec99f83aff1 // indirect
 	google.golang.org/protobuf v1.28.1
 )
+
+require (
+	golang.org/x/sys v0.2.0 // indirect
+	golang.org/x/xerrors v0.0.0-20200804184101-5ec99f83aff1 // indirect
+)

go.sum

@@ -48,7 +48,6 @@ golang.org/x/xerrors v0.0.0-20200804184101-5ec99f83aff1/go.mod h1:I/5z698sn9Ka8T
 google.golang.org/protobuf v1.26.0-rc.1/go.mod h1:jlhhOSvTdKEhbULTjvd4ARK9grFBp09yW+WbY/TyQbw=
 google.golang.org/protobuf v1.28.1 h1:d0NfwRgPtno5B1Wa6L2DAG+KivqkdutMf1UhdNx175w=
 google.golang.org/protobuf v1.28.1/go.mod h1:HV8QOd/L58Z+nl8r43ehVNZIU/HEI6OcFqwMG9pJV4I=
-gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
 gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=