build: upgrade go directive in go.mod to 1.17 (#2443)

This commit enables support for module graph pruning and lazy module
loading for projects that are at Go 1.17 or higher.

Reference: https://go.dev/ref/mod#go-mod-file-go
Reference: https://go.dev/ref/mod#graph-pruning
Reference: https://go.dev/ref/mod#lazy-loading
Signed-off-by: Eng Zer Jun <[email protected]>

Author: Juneezee

go.mod

@@ -1,6 +1,6 @@
 module github.com/grpc-ecosystem/grpc-gateway/v2
 
-go 1.14
+go 1.17
 
 require (
 	github.com/antihax/optional v1.0.0
@@ -14,3 +14,12 @@ require (
 	google.golang.org/protobuf v1.27.1
 	sigs.k8s.io/yaml v1.3.0
 )
+
+require (
+	golang.org/x/net v0.0.0-20210405180319-a5a99cb37ef4 // indirect
+	golang.org/x/sys v0.0.0-20210510120138-977fb7262007 // indirect
+	golang.org/x/text v0.3.5 // indirect
+	golang.org/x/xerrors v0.0.0-20200804184101-5ec99f83aff1 // indirect
+	google.golang.org/appengine v1.6.6 // indirect
+	gopkg.in/yaml.v2 v2.4.0 // indirect
+)