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.el

@@ -355,7 +355,6 @@ config.el instead."
       (when (boundp 'libgnutls-version)
         (concat "SECURE128:+SECURE192:-VERS-ALL"
                 (if (and (not IS-WINDOWS)
-                         (not (version< emacs-version "26.3"))
                          (>= libgnutls-version 30605))
                     ":+VERS-TLS1.3")
                 ":+VERS-TLS1.2"))