all: test with Go 1.20 and 1.21

Testing with Go 1.21 also requires updating staticcheck.

Also remove Go 1.13, 1.14, 1.15 and 1.16 from the version matrix
and describe the version policy in a comment.

Change-Id: I28093d405b26e8e34e17119bddc5c990321ec348
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/527895
Reviewed-by: Lasse Folger <lassefolger@google.com>
Reviewed-by: Damien Neil <dneil@google.com>

integration_test.go

@@ -38,22 +38,23 @@ var (
 	protobufVersion = "24.0-rc2"
 
 	golangVersions = func() []string {
-		var vers []string
-		switch runtime.GOOS + "/" + runtime.GOARCH {
-		case "darwin/arm64":
-		default:
-			vers = []string{"1.13.15", "1.14.15", "1.15.15"}
+		// Version policy: same version as is in the x/ repos' go.mod.
+		return []string{
+			"1.17.13",
+			"1.18.10",
+			"1.19.13",
+			"1.20.7",
+			"1.21.1",
 		}
-		return append(vers, "1.16.15", "1.17.13", "1.18.10", "1.19.6")
 	}()
 	golangLatest = golangVersions[len(golangVersions)-1]
 
-	staticcheckVersion = "2022.1.2"
+	staticcheckVersion = "2023.1.6"
 	staticcheckSHA256s = map[string]string{
-		"darwin/amd64": "baa35f8fb967ee2aacad57f026e3724fbf8d9b7ad8f682f4d44b2084a96e103b",
-		"darwin/arm64": "9f01a581eeea088d0a6272538360f6d84996d66ae554bfada8026fe24991daa0",
-		"linux/386":    "4cf74373e5d668b265d7a241b59ba7d26064f2cd6af50b77e62c2b3e2f3afb43",
-		"linux/amd64":  "6dbb7187e43812fa23363cdaaa90ab13544dd36e24d02e2347014e4cf265f06d",
+		"darwin/amd64": "b14a0cbd3c238713f5f9db41550893ea7d75d8d7822491c7f4e33e2fe43f6305",
+		"darwin/arm64": "f1c869abe6be2c6ab727dc9d6049766c947534766d71a1798c12a37526ea2b6f",
+		"linux/386":    "02859a7c44c7b5ab41a70d9b8107c01ab8d2c94075bae3d0b02157aff743ca42",
+		"linux/amd64":  "45337834da5dc7b8eff01cb6b3837e3759503cfbb8edf36b09e42f32bccb1f6e",
 	}
 
 	// purgeTimeout determines the maximum age of unused sub-directories.