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

Summary

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 (if anything) on GitLab Chart 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 some docs and in a yaml file:

./doc/advanced/external-gitaly/external-omnibus-gitaly.md:38:https://gitlab.com/gitlab-org/gitlab/blob/master/doc/administration/gitaly/index.md#gitaly-server-configuration
./doc/charts/gitlab/sidekiq/index.md:404:1. [`app/workers/all_queues.yml`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/workers/all_queues.yml)
./doc/charts/gitlab/sidekiq/index.md:405:1. [`ee/app/workers/all_queues.yml`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/workers/all_queues.yml)
./doc/charts/globals.md:1249:GitLab ships with a [default manifest file for Pseudonymizer](https://gitlab.com/gitlab-org/gitlab/blob/master/config/pseudonymizer.yml).
./doc/charts/globals.md:1296:sample [`gitlab.yml`](https://gitlab.com/gitlab-org/gitlab/blob/master/config/gitlab.yml.example#L346-427)
./.gitlab-ci.yml:499:  - wget https://gitlab.com/gitlab-org/gitlab/-/raw/master/scripts/trigger-build

Rough plan could be as follow:

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