Do foundational work to support to support gitlab-org/gitlab default branch

Overview

gitlab-com/gl-infra/delivery#1357 (closed) plans to rename gitlab-org/gitlab default branch from master to main, the timeline is still in the works (it might happen during the first quarter of 2021) but when it happens we should be prepared.

Let's use this issue to discuss what needs to be done on Gitaly before the switch.

Proposal

I'm not certain this project depends on a hardcoded master branch for gitlab-org/gitlab. From a quick search, seems it only references master on the docs, which makes me think this project is detached from gitlab-org/gitlab and does not depend on a hardcoded master branch.

./.gitlab/issue_templates/Gitaly Client Update.md:11:- [ ] update [GITLAB_SHELL_VERSION in gitlab](https://gitlab.com/gitlab-org/gitlab/-/blob/master/GITLAB_SHELL_VERSION) MR LINK
./.gitlab/issue_templates/Gitaly Client Update.md:12:- [ ] update [GITLAB_WORKHORSE_VERSION in gitlab](https://gitlab.com/gitlab-org/gitlab/-/blob/master/GITLAB_WORKHORSE_VERSION) MR LINK
./.gitlab/issue_templates/Gitaly Client Update.md:13:- [ ] update [GITLAB_ELASTICSEARCH_INDEXER_VERSION in gitlab](https://gitlab.com/gitlab-org/gitlab/-/blob/master/GITLAB_ELASTICSEARCH_INDEXER_VERSION) MR LINK
./doc/PROCESS.md:73:    git shortlog $(curl -s https://gitlab.com/gitlab-org/gitlab/-/raw/HASH/GITALY_SERVER_VERSION)..origin/master
./doc/README.md:9:GitLab's  [configuration documentation](https://gitlab.com/gitlab-org/gitlab/blob/master/doc/administration/gitaly/index.md).
./doc/README.md:11:The reference guide is documented in https://gitlab.com/gitlab-org/gitlab/blob/master/doc/administration/gitaly/reference.md.
./doc/object_quarantine.md:118:store"](https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/api/internal/base.rb#L43)

What we could do is:

1. Identify what areas of the Gitaly codebase depend on a hardcoded master branch for gitlab-org/gitlab
2. Adjust the corresponding files (if any)