conf: print warning that rust2rpm.conf file format is deprecated

rust2rpm/conf.py

@@ -552,6 +552,12 @@ def load_config(features: set[str], target: str) -> TomlConf:
         log.error(str(exc))
         sys.exit(1)
 
+    log.warn(
+        "The rust2rpm.conf file format is deprecated and support for it will "
+        + "be removed in a future release of rust2rpm. Please migrate settings "
+        + "to the new TOML based format (rust2rpm.toml)."
+    )
+
     tomlconf = TomlConf.from_conf(distconf)
     return tomlconf