Go back to grave quoting in Gnus

Go back to grave quoting in Gnus

* auth-source.el (auth-source-netrc-parse-entries):
* gnus-agent.el (gnus-agent-check-overview-buffer)
(gnus-agent-fetch-headers):
* gnus-int.el (gnus-start-news-server):
* gnus-registry.el (gnus-registry--split-fancy-with-parent-internal)
(gnus-registry-post-process-groups):
* gnus-score.el (gnus-summary-increase-score):
* gnus-start.el (gnus-convert-old-newsrc):
* gnus-topic.el (gnus-topic-rename):
* legacy-gnus-agent.el (gnus-agent-unlist-expire-days):
* spam.el (spam-check-blackholes):
Stick with grave quoting in diagnostics strings.  This is more
portable to older Emacs, desirable for Gnus.