Replace blacklist/whitelist in doc//user/gitlab_com/index.md

URL: https://docs.gitlab.com/ee/user/gitlab_com/index.html

grep -R -E 'whitelist|blacklist' doc/user/gitlab_com/index.md

doc/user/gitlab_com/index.md:from those IPs and whitelist them.
doc/user/gitlab_com/index.md:GitLab.com is fronted by Cloudflare. For incoming connections to GitLab.com you might need to whitelist CIDR blocks of Cloudflare ([IPv4](https://www.cloudflare.com/ips-v4) and [IPv6](https://www.cloudflare.com/ips-v6))