all: update to Go 1.20, use strings.CutPrefix/Suffix instead of our fork

Updates #7123
Updates #5309

Change-Id: I90bcd87a2fb85a91834a0dd4be6e03db08438672
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/debug.go

@@ -39,7 +39,6 @@ import (
 	"tailscale.com/types/key"
 	"tailscale.com/types/logger"
 	"tailscale.com/util/must"
-	"tailscale.com/util/strs"
 )
 
 var debugCmd = &ffcli.Command{
@@ -259,7 +258,7 @@ func runDebug(ctx context.Context, args []string) error {
 			e.Encode(wfs)
 			return nil
 		}
-		if name, ok := strs.CutPrefix(debugArgs.file, "delete:"); ok {
+		if name, ok := strings.CutPrefix(debugArgs.file, "delete:"); ok {
 			return localClient.DeleteWaitingFile(ctx, name)
 		}
 		rc, size, err := localClient.GetWaitingFile(ctx, debugArgs.file)

cmd/tailscale/cli/file.go

@@ -31,7 +31,6 @@ import (
 	"tailscale.com/net/tsaddr"
 	"tailscale.com/tailcfg"
 	"tailscale.com/util/quarantine"
-	"tailscale.com/util/strs"
 	"tailscale.com/version"
 )
 
@@ -90,7 +89,7 @@ func runCp(ctx context.Context, args []string) error {
 		return errors.New("usage: tailscale file cp <files...> <target>:")
 	}
 	files, target := args[:len(args)-1], args[len(args)-1]
-	target, ok := strs.CutSuffix(target, ":")
+	target, ok := strings.CutSuffix(target, ":")
 	if !ok {
 		return fmt.Errorf("final argument to 'tailscale file cp' must end in colon")
 	}

cmd/tailscale/cli/up.go

@@ -34,7 +34,6 @@ import (
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/logger"
 	"tailscale.com/types/preftype"
-	"tailscale.com/util/strs"
 	"tailscale.com/version"
 	"tailscale.com/version/distro"
 )
@@ -174,7 +173,7 @@ type upArgsT struct {
 
 func (a upArgsT) getAuthKey() (string, error) {
 	v := a.authKeyOrFile
-	if file, ok := strs.CutPrefix(v, "file:"); ok {
+	if file, ok := strings.CutPrefix(v, "file:"); ok {
 		b, err := os.ReadFile(file)
 		if err != nil {
 			return "", err