Upgrade Gitaly to Go v1.15

Prior to upgrading Gitaly to Go v1.15, any breaking behavior in the upgrade should be identified and mitigated. Once identified, they can be added to this checklist:

• x509 certs no longer honor the deprecated common-name field. Now the SAN field is required to do hostname verification. Gitaly documentation will need to be updated to mention this in https://docs.gitlab.com/ee/administration/gitaly/#enable-tls-support.