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>

util/hashx/block512_test.go

@@ -47,7 +47,7 @@ type hasher interface {
 
 func hashSuite(h hasher) {
 	for i := range 10 {
-		for j := 0; j < 10; j++ {
+		for range 10 {
 			h.HashUint8(0x01)
 			h.HashUint8(0x23)
 			h.HashUint32(0x456789ab)