Add EmitWarningsOnPlaceholders("age")

Added EmitWarningsOnPlaceholders("age") to warn when an undefined GUC parameter is set for a placeholder named "age".
Warnings are issued when PostgreSQL starts.

When PostgreSQL starts
```
2024-08-01 18:32:31.220 JST [2039799] WARNING: invalid configuration parameter name "age.invalid_parameter", removing it
```

src/backend/utils/ag_guc.c

@@ -42,4 +42,5 @@ void define_config_params(void)
                              NULL,
                              NULL,
                              NULL);
+    EmitWarningsOnPlaceholders("age");
 }