Add deprecation warning & restore test

lighthouse/tests/validator_client.rs

@@ -422,6 +422,12 @@ fn no_doppelganger_protection_flag() {
         .run()
         .with_config(|config| assert!(!config.enable_doppelganger_protection));
 }
+#[test]
+fn produce_block_v3_flag() {
+    // The flag is DEPRECATED but providing it should not trigger an error.
+    // We can delete this test when deleting the flag entirely.
+    CommandLineTest::new().flag("produce-block-v3", None).run();
+}
 
 #[test]
 fn no_gas_limit_flag() {

validator_client/src/config.rs

@@ -381,6 +381,14 @@ impl Config {
             config.prefer_builder_proposals = true;
         }
 
+        if cli_args.get_flag("produce-block-v3") {
+            warn!(
+                log,
+                "produce-block-v3 flag";
+                "note" => "deprecated flag has no effect and should be removed"
+            );
+        }
+
         config.gas_limit = cli_args
             .get_one::<String>("gas-limit")
             .map(|gas_limit| {