Update CHANGELOG.md for 8.13.8
Update CHANGELOG.md for 8.13.8

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