encode: deprecate encoding mode

While I'd like to keep it, I'm apparently the maintainer now, and I have
no idea what the heck some of this code does, so it's deprecated.

DOCS/interface-changes.rst

@@ -56,6 +56,7 @@ Interface changes
     - add playlist-current-pos, playlist-playing-pos properties
     - Lua end-file events do not set the "error" field anymore, use "file_error"
       instead.
+    - deprecate encoding mode (lack of maintainer)
  --- mpv 0.32.0 ---
     - change behavior when using legacy option syntax with options that start
       with two dashes (``--`` instead of a ``-``). Now, using the recommended

common/encode_lavc.c

@@ -77,7 +77,8 @@ struct mux_stream {
 #define OPT_BASE_STRUCT struct encode_opts
 const struct m_sub_options encode_config = {
     .opts = (const m_option_t[]) {
-        {"o", OPT_STRING(file), .flags = CONF_NOCFG | CONF_PRE_PARSE | M_OPT_FILE},
+        {"o", OPT_STRING(file), .flags = CONF_NOCFG | CONF_PRE_PARSE | M_OPT_FILE,
+            .deprecation_message = "lack of maintainer"},
         {"of", OPT_STRING(format)},
         {"ofopts", OPT_KEYVALUELIST(fopts), .flags = M_OPT_HAVE_HELP},
         {"ovc", OPT_STRING(vcodec)},