fix: correct namecheap certbot plugin name from dns-namecheap to namecheap

- The certbot-dns-namecheap plugin uses --namecheap not --dns-namecheap
- This fixes the 'unrecognized arguments' error when obtaining certificates

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: Leechael

custom-domain/dstack-ingress/scripts/dns_providers/namecheap.py

@@ -11,7 +11,7 @@ class NamecheapDNSProvider(DNSProvider):
     """Namecheap DNS provider implementation."""
 
     DETECT_ENV = "NAMECHEAP_API_KEY"
-    CERTBOT_PLUGIN = "dns-namecheap"
+    CERTBOT_PLUGIN = "namecheap"
     CERTBOT_PACKAGE = "certbot-dns-namecheap"
     CERTBOT_PROPAGATION_SECONDS = 120
     CERTBOT_CREDENTIALS_FILE = "~/.namecheap/namecheap.ini"