Eglot: improve diagnostic-reporting performance

Eglot: improve diagnostic-reporting performance

After a change in the buffer has occured, it is often the case
that Flymake is quicker to ask for diagnostics than the server
is to supply them to us.  If we're still stuck with old outdated
diagnostics, don't forward them to Flymake, even if it eagerly
asks us for them.

* etc/EGLOT-NEWS (Changes in upcoming Eglot): Announce changes.

* lisp/progmodes/eglot.el
(eglot--diagnostics): Rework.
(eglot--report-to-flymake): Also take version.
(eglot-handle-notification textDocument/publishDiagnostics)
(eglot--managed-mode)
(eglot-flymake-backend): Tweak call to eglot--report-to-flymake.

(cherry picked from commit 7ae275f04c46e1724fafd8c8aa2f3eed5771df1c)