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>

derp/derphttp/derphttp_client.go

@@ -23,6 +23,7 @@ import (
 	"net/netip"
 	"net/url"
 	"runtime"
+	"strings"
 	"sync"
 	"time"
 
@@ -37,7 +38,6 @@ import (
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/key"
 	"tailscale.com/types/logger"
-	"tailscale.com/util/strs"
 )
 
 // Client is a DERP-over-HTTP client.
@@ -1028,7 +1028,7 @@ var ErrClientClosed = errors.New("derphttp.Client closed")
 func parseMetaCert(certs []*x509.Certificate) (serverPub key.NodePublic, serverProtoVersion int) {
 	for _, cert := range certs {
 		// Look for derpkey prefix added by initMetacert() on the server side.
-		if pubHex, ok := strs.CutPrefix(cert.Subject.CommonName, "derpkey"); ok {
+		if pubHex, ok := strings.CutPrefix(cert.Subject.CommonName, "derpkey"); ok {
 			var err error
 			serverPub, err = key.ParseNodePublicUntyped(mem.S(pubHex))
 			if err == nil && cert.SerialNumber.BitLen() <= 8 { // supports up to version 255