[management, client] Fix SSH server audience validator (#5105)

* **New Features**
  * SSH server JWT validation now accepts multiple audiences with backward-compatible handling of the previous single-audience setting and a guard ensuring at least one audience is configured.
* **Tests**
  * Test suites updated and new tests added to cover multiple-audience scenarios and compatibility with existing behavior.
* **Other**
  * Startup logging enhanced to report configured audiences for JWT auth.

client/ssh/proxy/proxy_test.go

@@ -132,7 +132,7 @@ func TestSSHProxy_Connect(t *testing.T) {
 		HostKeyPEM: hostKey,
 		JWT: &server.JWTConfig{
 			Issuer:       issuer,
-			Audience:     audience,
+			Audiences:    []string{audience},
 			KeysLocation: jwksURL,
 		},
 	}