all: set Go language version to Go 1.21

Go 1.21 is the oldest currently supported version of Go, see
https://go.dev/doc/devel/release#policy

All supported Go versions (1.21 and 1.22) support
forward compatibility and toolchain management, see
https://go.dev/blog/toolchain

People stuck on much older versions of Go should
stick to older versions of Go Protobuf, too.

fixes golang/protobuf#1613

Change-Id: Id997efd8b47949e82d073c1d26a51d27620f4b82
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/585095
Reviewed-by: Lasse Folger <lassefolger@google.com>
Auto-Submit: Michael Stapelberg <stapelberg@google.com>
Reviewed-by: Cassondra Foesch <cfoesch@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

go.mod

@@ -1,6 +1,6 @@
 module google.golang.org/protobuf
 
-go 1.17
+go 1.21
 
 require (
 	github.com/golang/protobuf v1.5.0

integration_test.go

@@ -37,13 +37,10 @@ var (
 	protobufVersion = "27.0-rc1"
 
 	golangVersions = func() []string {
-		// Version policy: same version as is in the x/ repos' go.mod.
+		// Version policy: oldest supported version of Go.
 		return []string{
-			"1.17.13",
-			"1.18.10",
-			"1.19.13",
-			"1.20.12",
-			"1.21.5",
+			"1.21.10",
+			"1.22.3",
 		}
 	}()
 	golangLatest = golangVersions[len(golangVersions)-1]