all: use Go 1.26 things, run most gofix modernizers

I omitted a lot of the min/max modernizers because they didn't
result in more clear code.

Some of it's older "for x := range 123".

Also: errors.AsType, any, fmt.Appendf, etc.

Updates #18682

Change-Id: I83a451577f33877f962766a5b65ce86f7696471c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/tstun/wrap.go

@@ -1406,11 +1406,11 @@ func (t *Wrapper) InjectInboundPacketBuffer(pkt *netstack_PacketBuffer, buffs []
 			return err
 		}
 	}
-	for i := 0; i < n; i++ {
+	for i := range n {
 		buffs[i] = buffs[i][:PacketStartOffset+sizes[i]]
 	}
 	defer func() {
-		for i := 0; i < n; i++ {
+		for i := range n {
 			buffs[i] = buffs[i][:cap(buffs[i])]
 		}
 	}()