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/set/intset.go

@@ -152,7 +152,7 @@ func (s bitSet) values() iter.Seq[uint64] {
 	return func(yield func(uint64) bool) {
 		// Hyrum-proofing: randomly iterate in forwards or reverse.
 		if rand.Uint64()%2 == 0 {
-			for i := 0; i < bits.UintSize; i++ {
+			for i := range bits.UintSize {
 				if s.contains(uint64(i)) && !yield(uint64(i)) {
 					return
 				}