Factor out *scratch* initialization

Factor out *scratch* initialization

* lisp/simple.el (get-scratch-buffer-create): New function, factored
out of scratch-buffer, and additionally clearing the modification flag
and calling substitute-command-keys (bug#55257).
(scratch-buffer):
* lisp/server.el (server-execute):
* lisp/startup.el (normal-no-mouse-startup-screen, command-line-1):
* lisp/window.el (last-buffer, window-normalize-buffer-to-switch-to):
* src/buffer.c (Fother_buffer, other_buffer_safely): Use it.
(syms_of_buffer): Add Qget_scratch_buffer_create.
* lisp/startup.el (startup--get-buffer-create-scratch): Delete
now-unused function.
* doc/lispref/os.texi (Summary: Sequence of Actions at Startup):
* NEWS (Incompatible changes in Emacs 29.1): Document the change.