Upgrade grpc gem to 1.15.x

• gitlab-ce: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22350
• gitaly-ruby: gitaly!918 (merged)

Follow-up to https://gitlab.com/gitlab-org/gitlab-ce/issues/39928

mentioned in issue #39928 (closed)

@jramsay I randomly assigned 11.5 because this is not a lot of work and it is a good habit to keep up with updates to grpc. It's been a long time since we last upgraded anyway (1bfaf102).

assigned to @alejandro

mentioned in merge request gitaly!918 (merged)

mentioned in merge request !22350 (merged)

changed the description

marked the checklist item gitlab-ce: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22350 as completed

marked the checklist item gitaly-ruby: gitaly!918 (merged) as completed

Closed by !22350 (merged) and gitaly!918 (merged)

closed

added devopscreate groupgitaly labels

removed 1 deleted label