Use 'C-x t' for tab-prefix-map instead of 'C-x 6' still used by 2C-mode-map.

Use 'C-x t' for tab-prefix-map instead of 'C-x 6' still used by 2C-mode-map.

* doc/emacs/commands.texi (Keys): Add 'C-x t'.
* doc/emacs/frames.texi (Tab Bars): Replace 'C-x 6' with 'C-x t'.
* doc/lispref/keymaps.texi (Prefix Keys): Add 'C-x t' (tab-prefix-map).
* doc/lispref/maps.texi (Standard Keymaps): Add tab-prefix-map, tab-bar-map.

* lisp/dired.el: Rebind 'C-x 6 d' to 'C-x t d'.  Use tab-prefix-map.

* lisp/subr.el (ctl-x-6-map, ctl-x-6-prefix, ctl-x-t-prefix): Remove.
(tab-prefix-map): New keymap bound to 'C-x t'.

* lisp/tab-bar.el: Use tab-prefix-map instead of ctl-x-6-map.

* lisp/textmodes/two-column.el: Restore autoload global setting of
"\C-x6" to 2C-command.

* lisp/ldefs-boot.el: Update to add setting "\C-x6" to 2C-command.

* doc/emacs/commands.texi (Keys):
* doc/emacs/text.texi (Two-Column):
* doc/lispref/keymaps.texi (Prefix Keys):
* doc/lispref/maps.texi (Standard Keymaps):
Restore 'C-x 6' bound to '2C-mode-map'.