Update stale "master" branch references to "main".

We renamed master->main a long time ago.

Author: mateuszmandera

README.md

@@ -427,5 +427,5 @@ make a high-quality Docker image for Zulip possible.
 [install-normal]: https://zulip.readthedocs.io/en/latest/production/install.html#installer-options
 [outgoing-email]: https://zulip.readthedocs.io/en/latest/production/email.html
 [server-settings]: https://zulip.readthedocs.io/en/latest/production/settings.html
-[prod-settings-template]: https://github.com/zulip/zulip/blob/master/zproject/prod_settings_template.py
+[prod-settings-template]: https://github.com/zulip/zulip/blob/main/zproject/prod_settings_template.py
 [create-organization]: http://zulip.readthedocs.io/en/latest/production/install.html#step-3-create-a-zulip-organization-and-log-in

UPGRADING.md

@@ -85,11 +85,11 @@ docker-compose exec -u zulip zulip cat /home/zulip/deployments/current/version.p
        args:
          # Change these if you want to build zulip from a different repo/branch
          ZULIP_GIT_URL: https://github.com/zulip/zulip.git
-         ZULIP_GIT_REF: master
+         ZULIP_GIT_REF: main
    ```
 
    You can set `ZULIP_GIT_URL` to any clone of the zulip/zulip git repository,
-   and `ZULIP_GIT_REF` to be any ref name in that repository (e.g. `master` or
+   and `ZULIP_GIT_REF` to be any ref name in that repository (e.g. `main` or
    `1.9.0` or `445932cc8613c77ced023125248c8b966b3b7528`).
 
 2. Run `docker-compose build zulip` to build a Zulip Docker image from the

kubernetes/chart/zulip/values-local.yaml.example

@@ -7,7 +7,7 @@ zulip:
   # password: set-secure-password
 
   # Add any other environment variables you want to configure here.
-  # based on; https://github.com/zulip/docker-zulip/blob/master/docker-compose.yml#L63
+  # based on; https://github.com/zulip/docker-zulip/blob/main/docker-compose.yml#L63
   # these values will be merged with db secrets and hosts/ports
   environment:
     # Domain

kubernetes/chart/zulip/values.yaml

@@ -125,7 +125,7 @@ affinity: {}
 
 zulip:
   ## Environment variables based on
-  ## https://github.com/zulip/docker-zulip/blob/master/docker-compose.yml#L63
+  ## https://github.com/zulip/docker-zulip/blob/main/docker-compose.yml#L63
   environment:
     ## Disable the default Zulip requirement of HTTPS.  HTTPS and
     ## certificates are managed by the Kubernetes cluster, so by