Eglot: abandon compat.el

Eglot: abandon compat.el

After period of months using this to help compatibility to older Emacs
versions.  It's not really buying us that much and complicates
maintenance, so it gets the axe.

A common problem is the compatibility alias for require-with-check,
which often errors with some "Feature X loaded from Y is now provided by
Z".  The tests for Emacs 26.3 were all failing in the Github tracker
ever since this (and track-changes.el) were added to Eglot.

* lisp/progmodes/eglot.el (compat): No longer require.
(eglot-alternatives): Reword comment.
(eglot-alternatives): Call eglot--executable-find.
(eglot--guess-contact): Call eglot--executable-find.
(eglot--connect): Check if package-get-version exists.
(eglot--format): Check if substitute-quotes exists.
(eglot--format-markup): Check if text-property-search exists.

(cherry picked from commit a60249b098d22093ec79b6cbc27a759b7b4f8883)