CHANGELOG: split into separate files per major

The changelog as it was, was by now > 8000 lines long and becoming unwieldy.

This commit splits the changelog file up into separate files per major release series.

It also adds the older changelog files to the `.gitattributes` file, so only the changelog files for the current and the previous major will be included in release zips.

Author: jrfnl

.gitattributes

@@ -18,6 +18,9 @@ tests/EndToEnd/**/                    export-ignore
 .remarkrc                             export-ignore
 .shellcheckrc                         export-ignore
 .yamllint.yml                         export-ignore
+CHANGELOG-0.x.md                      export-ignore
+CHANGELOG-1.x.md                      export-ignore
+CHANGELOG-2.x.md                      export-ignore
 phpcs.xml.dist                        export-ignore
 phpstan.neon.dist                     export-ignore
 phpunit.xml.dist                      export-ignore