Drop Emacs 26.x support

Emacs 27.x has been the stable version of Emacs for nearly a year, and
introduces a litany of bugfixes, performance, and quality-of-life
improvements that significantly reduce Doom's maintenance burden (like
XDG support, early-init.el, image manipulation without imagemagick, a
native JSON library, harfbuzz support, pdumper, and others).

With so many big changes on Doom's horizon, I like having one less (big)
thing to worry about.

Also reverts bb677cf7a (#5232) as it is no longer needed.

core/core-lib.el

@@ -625,72 +625,7 @@ testing advice (when combined with `rotate-text').
 ;;
 ;;; Backports
 
-(eval-when! (version< emacs-version "27.0.90")
-  ;; DEPRECATED Backported from Emacs 27
-  (defmacro setq-local (&rest pairs)
-    "Make variables in PAIRS buffer-local and assign them the corresponding values.
-
-PAIRS is a list of variable/value pairs.  For each variable, make
-it buffer-local and assign it the corresponding value.  The
-variables are literal symbols and should not be quoted.
-
-The second VALUE is not computed until after the first VARIABLE
-is set, and so on; each VALUE can use the new value of variables
-set earlier in the ‘setq-local’.  The return value of the
-‘setq-local’ form is the value of the last VALUE.
-
-\(fn [VARIABLE VALUE]...)"
-    (declare (debug setq))
-    (unless (zerop (mod (length pairs) 2))
-      (error "PAIRS must have an even number of variable/value members"))
-    (let ((expr nil))
-      (while pairs
-        (unless (symbolp (car pairs))
-          (error "Attempting to set a non-symbol: %s" (car pairs)))
-        ;; Can't use backquote here, it's too early in the bootstrap.
-        (setq expr
-              (cons
-               (list 'set
-                     (list 'make-local-variable (list 'quote (car pairs)))
-                     (car (cdr pairs)))
-               expr))
-        (setq pairs (cdr (cdr pairs))))
-      (macroexp-progn (nreverse expr)))))
-
-(eval-when! (version< emacs-version "27.1")
-  ;; DEPRECATED Backported from Emacs 27. Remove when 26.x support is dropped.
-  (defun executable-find (command &optional remote)
-    "Search for COMMAND in `exec-path' and return the absolute file name.
-Return nil if COMMAND is not found anywhere in `exec-path'.  If
-REMOTE is non-nil, search on the remote host indicated by
-`default-directory' instead."
-    (if (and remote (file-remote-p default-directory))
-        (let ((res (locate-file
-                    command
-                    (mapcar
-                     (lambda (x) (concat (file-remote-p default-directory) x))
-                     (exec-path))
-                    exec-suffixes 'file-executable-p)))
-          (when (stringp res) (file-local-name res)))
-      ;; Use 1 rather than file-executable-p to better match the
-      ;; behavior of call-process.
-      (let ((default-directory
-              (let (file-name-handler-alist)
-                (file-name-quote default-directory))))
-        (locate-file command exec-path exec-suffixes 1)))))
-
-(eval-when! (not (fboundp 'exec-path))
-  ;; DEPRECATED Backported from Emacs 27.1. Remove when 26.x support is dropped.
-  (defun exec-path ()
-    "Return list of directories to search programs to run in remote subprocesses.
-The remote host is identified by `default-directory'.  For remote
-hosts that do not support subprocesses, this returns `nil'.
-If `default-directory' is a local directory, this function returns
-the value of the variable `exec-path'."
-    (let ((handler (find-file-name-handler default-directory 'exec-path)))
-      (if handler
-          (funcall handler 'exec-path)
-        exec-path))))
+;; None at the moment!
 
 (provide 'core-lib)
 ;;; core-lib.el ends here