cmd/k8s-operator,ipn/ipnlocal: allow opting out of ACME order replace extension (#18252)

In dynamically changing environments where ACME account keys and certs
are stored separately, it can happen that the account key would get
deleted (and recreated) between issuances. If that is the case,
we currently fail renewals and the only way to recover is for users
to delete certs.
This adds a config knob to allow opting out of the replaces extension
and utilizes it in the Kubernetes operator where there are known
user workflows that could end up with this edge case.

Updates #18251

Signed-off-by: Irbe Krumina <irbe@tailscale.com>

ipn/ipnlocal/cert.go

@@ -551,8 +551,11 @@ var getCertPEM = func(ctx context.Context, b *LocalBackend, cs certStore, logf l
 	// If we have a previous cert, include it in the order. Assuming we're
 	// within the ARI renewal window this should exclude us from LE rate
 	// limits.
+	// Note that this order extension will fail renewals if the ACME account key has changed
+	// since the last issuance, see
+	// https://github.com/tailscale/tailscale/issues/18251
 	var opts []acme.OrderOption
-	if previous != nil {
+	if previous != nil && !envknob.Bool("TS_DEBUG_ACME_FORCE_RENEWAL") {
 		prevCrt, err := previous.parseCertificate()
 		if err == nil {
 			opts = append(opts, acme.WithOrderReplacesCert(prevCrt))