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>

ipn/auditlog/auditlog.go

@@ -69,8 +69,11 @@ type Opts struct {
 // IsRetryableError returns true if the given error is retryable
 // See [controlclient.apiResponseError].  Potentially retryable errors implement the Retryable() method.
 func IsRetryableError(err error) bool {
-	var retryable interface{ Retryable() bool }
-	return errors.As(err, &retryable) && retryable.Retryable()
+	retryable, ok := errors.AsType[interface {
+		error
+		Retryable() bool
+	}](err)
+	return ok && retryable.Retryable()
 }
 
 type backoffOpts struct {