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/vizerror/vizerror.go

@@ -77,6 +77,5 @@ func WrapWithMessage(wrapped error, publicMsg string) error {
 
 // As returns the first vizerror.Error in err's chain.
 func As(err error) (e Error, ok bool) {
-	ok = errors.As(err, &e)
-	return
+	return errors.AsType[Error](err)
 }