patch 8.2.2531: Vim9: the :k command is obscure

Problem:    Vim9: the :k command is obscure.
Solution:   Disallow using :k, can use :mark instead. (closes #7874)

src/vim9compile.c

@@ -8520,6 +8520,7 @@ compile_def_function(
 	    case CMD_append:
 	    case CMD_change:
 	    case CMD_insert:
+	    case CMD_k:
 	    case CMD_t:
 	    case CMD_xit:
 		    not_in_vim9(&ea);