Unbreak Eglot (as a :core ELPA package) on Emacs 26.3

Unbreak Eglot (as a :core ELPA package) on Emacs 26.3

Because of the lack of regular automated testing on a CI system, many
recent developments in and outside Eglot had wrecked this
compatibility.

The GitHub CI available at https://github.com/joaotavora/eglot/actions
can be used to run this combination of eglot.el + eglot-tests.el on
26.3, 27.2 and 28.2.

* etc/EGLOT-NEWS: Mention new version.

* lisp/progmodes/eglot.el (eglot--reporter-update): New
compatibility shim.
(eglot-handle-notification $/progress): Use it.
(eglot-handle-notification textDocument/publishDiagnostics): Use
two-arg assoc-delete-all.
(Version): Bump to 1.13.

* test/lisp/progmodes/eglot-tests.el
(Commentary): Tweak.
(tramp): Tweak require;
(eglot-test-diagnostic-tags-unnecessary-code): Use
jsonrpc--encode.
(eglot--call-with-tramp-test): Adjust dependency on
ert-remote-temporary-file-directory.
(eglot-test-rust-on-type-formatting)
(eglot-test-project-wide-diagnostics-rust-analyzer): Wait longer.