derp, net/dns/recursive: use Go 1.21 min

Updates #8419

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

derp/derp.go

@@ -199,7 +199,7 @@ func readFrame(br *bufio.Reader, maxSize uint32, b []byte) (t frameType, frameLe
 		return 0, 0, fmt.Errorf("frame header size %d exceeds reader limit of %d", frameLen, maxSize)
 	}
 
-	n, err := io.ReadFull(br, b[:minUint32(frameLen, uint32(len(b)))])
+	n, err := io.ReadFull(br, b[:min(frameLen, uint32(len(b)))])
 	if err != nil {
 		return 0, 0, err
 	}
@@ -233,10 +233,3 @@ func writeFrame(bw *bufio.Writer, t frameType, b []byte) error {
 	}
 	return bw.Flush()
 }
-
-func minUint32(a, b uint32) uint32 {
-	if a < b {
-		return a
-	}
-	return b
-}

net/dns/recursive/recursive.go

@@ -14,7 +14,6 @@ import (
 	"time"
 
 	"github.com/miekg/dns"
-	"golang.org/x/exp/constraints"
 	"golang.org/x/exp/slices"
 	"tailscale.com/envknob"
 	"tailscale.com/net/netns"
@@ -484,13 +483,6 @@ func (r *Resolver) resolveRecursive(
 	return nil, 0, ErrNoResponses
 }
 
-func min[T constraints.Ordered](a, b T) T {
-	if a < b {
-		return a
-	}
-	return b
-}
-
 // queryNameserver sends a query for "name" to the nameserver "nameserver" for
 // records of type "qtype", trying both UDP and TCP connections as
 // appropriate.