Allow Github PR link in CHANGES.html [LUCENE-10545]

[asfimport]

changes2html already supports links to Github PRs (#6446), but the link is obsoleted so a small modification is needed to make it work again.

With this change, developers can directly link to their pull requests from CHANGES.txt, skipping creating Jira issues.

When the change made (2014) Github was still a "third party" to Apache and the integration had been on experimental status; the situation was changed - now GitHub is the first-class code hosting service for ASF projects. I think the requirement to open a Jira issue can be relaxed.

---

Migrated from LUCENE-10545 by Tomoko Uchida (@mocobeta), resolved May 02 2022
Parent: #11579

[asfimport]

Tomoko Uchida (@mocobeta) (migrated from JIRA)

This PR shows a concrete example to show how it'll work.

#854

[asfimport]

Tomoko Uchida (@mocobeta) (migrated from JIRA)

Note that this does not replace Jira issue with Github (we still absolutely need issues for discussion), but provides an option for developers who already have working patches.

[asfimport]

Alan Woodward (@romseygeek) (migrated from JIRA)

Bulk close for 9.2.0 release