patch 8.2.1447: Vim9: return type of keys() is list<any>

Problem:    Vim9: return type of keys() is list<any>.
Solution:   Should be list<string>. (closes #6711)
diff --git a/src/evalfunc.c b/src/evalfunc.c
index d95a1db..ff81462 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -715,7 +715,7 @@
     {"js_encode",	1, 1, FEARG_1,	  ret_string,	f_js_encode},
     {"json_decode",	1, 1, FEARG_1,	  ret_any,	f_json_decode},
     {"json_encode",	1, 1, FEARG_1,	  ret_string,	f_json_encode},
-    {"keys",		1, 1, FEARG_1,	  ret_list_any,	f_keys},
+    {"keys",		1, 1, FEARG_1,	  ret_list_string, f_keys},
     {"last_buffer_nr",	0, 0, 0,	  ret_number,	f_last_buffer_nr}, // obsolete
     {"len",		1, 1, FEARG_1,	  ret_number,	f_len},
     {"libcall",		3, 3, FEARG_3,	  ret_string,	f_libcall},