Make cl-gensym obsolete in favor of built-in gensym

Make cl-gensym obsolete in favor of built-in gensym

* lisp/emacs-lisp/cl-macs.el (cl-gensym): Declare function
obsolete in favor of gensym, added in Emacs 26.1.  The only reason
for its existence is that it allows an integer argument, but
that's not really useful, so it's better to remove this complexity.
Ref: https://lists.gnu.org/r/emacs-devel/2017-09/msg00313.html
* doc/misc/cl.texi (Symbols, Creating Symbols, Efficiency Concerns)
(Obsolete Setf Customization): Don't document above obsolete function.
* lisp/emacs-lisp/cl-macs.el (cl--parse-loop-clause):
* lisp/emacs-lisp/edebug.el (edebug-make-form-wrapper):
* lisp/obsolete/cl.el (cl--function-convert, lexical-let):
* lisp/obsolete/thumbs.el (thumbs-temp-file):
* lisp/progmodes/eglot.el (eglot--lambda)
(eglot--when-live-buffer, eglot--when-buffer-window)
(eglot--collecting-xrefs, eglot--glob-parse):
* lisp/progmodes/flymake.el (flymake--run-backend):
* test/lisp/emacs-lisp/package-tests.el (with-package-test):
* test/lisp/progmodes/eglot-tests.el (eglot--guessing-contact):
* test/lisp/progmodes/elisp-mode-tests.el
(elisp-shorthand-read-buffer, elisp-shorthand-read-from-string): Prefer
plain gensym to cl-gensym in files that can depend on Emacs 26.1.
* lisp/jsonrpc.el (jsonrpc-lambda, jsonrpc-request): Prefer gensym to
cl-gensym only when defined, as this file supports Emacs 25.1
* test/lisp/emacs-lisp/cl-macs-tests.el (cl-lib-test-gensym): Simplify
test as 'should' no longer uses cl-gensym.

(cherry picked from commit 60b071e224136207f7fa24983037522e637e7efa)