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/goroutines/goroutines.go

@@ -52,7 +52,7 @@ func scrubHex(buf []byte) []byte {
 			in[0] = '?'
 			return
 		}
-		v := []byte(fmt.Sprintf("v%d%%%d", len(saw)+1, u64%8))
+		v := fmt.Appendf(nil, "v%d%%%d", len(saw)+1, u64%8)
 		saw[inStr] = v
 		copy(in, v)
 	})