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/topk/topk_test.go

@@ -43,7 +43,7 @@ func TestTopK(t *testing.T) {
 		got  []int
 		want = []int{5, 6, 7, 8, 9}
 	)
-	for try := 0; try < 10; try++ {
+	for range 10 {
 		topk := NewWithParams[int](5, func(in []byte, val int) []byte {
 			return binary.LittleEndian.AppendUint64(in, uint64(val))
 		}, 4, 1000)