Remove Emacs 27 compatibility from Tramp

Remove Emacs 27 compatibility from Tramp

* doc/misc/tramp.texi (Frequently Asked Questions):
Adapt supported Emacs versions.

* doc/misc/trampver.texi:
* lisp/net/trampver.el: Change version to "2.8.0-pre".

* lisp/net/tramp-compat.el (tramp-compat-file-modes)
(tramp-compat-set-file-modes, tramp-compat-set-file-times)
(tramp-compat-directory-files)
(tramp-compat-directory-files-and-attributes)
(tramp-compat-directory-empty-p, tramp-compat-null-device)
(tramp-compat-string-replace, tramp-compat-string-search)
(tramp-compat-make-lock-file-name)
(tramp-compat-file-name-concat)
(tramp-compat-replace-regexp-in-region, tramp-compat-length<)
(tramp-compat-length>, tramp-compat-length=)
(tramp-compat-always): Remove.

* lisp/net/tramp-adb.el:
* lisp/net/tramp-androidsu.el:
* lisp/net/tramp-archive.el:
* lisp/net/tramp-cache.el:
* lisp/net/tramp-cmds.el:
* lisp/net/tramp-compat.el:
* lisp/net/tramp-crypt.el:
* lisp/net/tramp-fuse.el:
* lisp/net/tramp-gvfs.el:
* lisp/net/tramp-message.el:
* lisp/net/tramp-sh.el:
* lisp/net/tramp-smb.el:
* lisp/net/tramp-sshfs.el:
* lisp/net/tramp-sudoedit.el:
* lisp/net/tramp.el: Use original objects instead of their
`tramp-compat-' counterparts when possible.

(cherry picked from commit c4e8112f983a2add3253e5607f0c29f70e5b4081)