fix: trim trailing dots from certificate SANs

Trailing dots are not supposed to be in the cert SANs, but most
implementations allow it. Go 1.25.2 introduced strict validation for DNS
names in cert SANs, which leads to CoreDNS issue on GCP: as CoreDNS
1.13.1 was build with Go 1.25.2, it rejects a certSAN:

```
tls: failed to parse certificate from server: x509: SAN dNSName is malformed
```

The FQDN on GCP looks like: `<vm>.c.project.internal.` (note trailing
dot). Trim trailing dots when building SANs on all levels.

Signed-off-by: Andrey Smirnov <andrey.smirnov@siderolabs.com>
(cherry picked from commit e202b1f)

Author: smira

pkg/machinery/resources/secrets/cert_sans.go

@@ -8,6 +8,7 @@ import (
 	"net"
 	"net/netip"
 	"slices"
+	"strings"
 
 	"github.com/cosi-project/runtime/pkg/resource"
 	"github.com/cosi-project/runtime/pkg/resource/meta"
@@ -95,9 +96,12 @@ func (spec *CertSANSpec) AppendIPs(ips ...netip.Addr) {
 // AppendDNSNames skipping duplicates.
 func (spec *CertSANSpec) AppendDNSNames(dnsNames ...string) {
 	for _, dnsName := range dnsNames {
+		// remove trailing dot from the DNS name, as it shouldn't be stored in the cert SANs
+		dnsName = strings.TrimRight(dnsName, ".")
+
 		found := slices.Contains(spec.DNSNames, dnsName)
 
-		if !found {
+		if !found && dnsName != "" {
 			spec.DNSNames = append(spec.DNSNames, dnsName)
 		}
 	}