configure union merge for Changelog*.next.asciidoc (elastic#10003)

* configure union merge for Changelog*.next.asciidoc

This change automatically merges Changelog*.next.asciidoc files when
merging or rebasing via `merge=union`. No more need to fix changelog
files when doing `git rebase master`.

* Remove union merge from Changelog.asciidoc

(cherry picked from commit 05d7f5e)

.gitattributes

@@ -1,5 +1,5 @@
-CHANGELOG.md  merge=union
-CHANGELOG.asciidoc  merge=union
+CHANGELOG.next.asciidoc  merge=union
+CHANGELOG-developer.next.asciidoc  merge=union
 
 # Keep these file types as CRLF (Windows).
 *.bat    text eol=crlf