Update CHANGELOG.md for 8.15.2
Update CHANGELOG.md for 8.15.2

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