fix: upgrade Go from 1.25.5 to 1.25.6 to patch CVE-2025-61726, CVE-2025-61728, CVE-2025-61730 (#1992)

Upgrade Go version from 1.25.5 to 1.25.6 to fix multiple CVEs in the standard library:

- CVE-2025-61726: net/url package doesn't set a limit on query parameters (DoS)
- CVE-2025-61728: archive/zip uses super-linear file name indexing (DoS)
- CVE-2025-61730: TLS 1.3 handshake boundary issue (Information Disclosure)

Verification: go build completed successfully

Author: shibd

go.mod

@@ -1,6 +1,6 @@
 module github.com/streamnative/pulsarctl
 
-go 1.25.5
+go 1.25.6
 
 require (
 	github.com/apache/pulsar-client-go v0.18.0-candidate-1.0.20251222030102-3bb7d4eff361