Update CHANGELOG.md for 8.13.9
Update CHANGELOG.md for 8.13.9

[ci skip]
mmkassem/gitlabhq,LUMC/gitlabhq,dreampet/gitlab,dreampet/gitlab,dplarson/gitlabhq,stoplightio/gitlabhq,jirutka/gitlabhq,dplarson/gitlabhq,iiet/iiet-git,darkrasid/gitlabhq,dreampet/gitlab,openwide-java/gitlabhq,darkrasid/gitlabhq,htve/GitlabForChinese,jirutka/gitlabhq,LUMC/gitlabhq,LUMC/gitlabhq,mmkassem/gitlabhq,axilleas/gitlabhq,darkrasid/gitlabhq,axilleas/gitlabhq,stoplightio/gitlabhq,openwide-java/gitlabhq,htve/GitlabForChinese,dplarson/gitlabhq,iiet/iiet-git,t-zuehlsdorff/gitlabhq,htve/GitlabForChinese,stoplightio/gitlabhq,openwide-java/gitlabhq,htve/GitlabForChinese,mmkassem/gitlabhq,iiet/iiet-git,axilleas/gitlabhq,axilleas/gitlabhq,LUMC/gitlabhq,t-zuehlsdorff/gitlabhq,stoplightio/gitlabhq,mmkassem/gitlabhq,dreampet/gitlab,dplarson/gitlabhq,t-zuehlsdorff/gitlabhq,iiet/iiet-git,darkrasid/gitlabhq,jirutka/gitlabhq,t-zuehlsdorff/gitlabhq,jirutka/gitlabhq,openwide-java/gitlabhq