Migrate master branch to main

We're a few years behind the curve on this one, but using "master" as a
programming term is a callous practice that explicitly uses the
historical institution of slavery as a cheap, racist metaphor. Switch to
using "main", as it's the new default in git and GitHub.

certbot/docs/contributing.rst

@@ -258,8 +258,8 @@ certificate once it is issued. Some plugins, like the built-in Apache and Nginx
 plugins, implement both interfaces and perform both tasks. Others, like the
 built-in Standalone authenticator, implement just one interface.
 
-.. _interfaces.py: https://github.com/certbot/certbot/blob/master/certbot/certbot/interfaces.py
-.. _plugins/common.py: https://github.com/certbot/certbot/blob/master/certbot/certbot/plugins/common.py#L45
+.. _interfaces.py: https://github.com/certbot/certbot/blob/main/certbot/certbot/interfaces.py
+.. _plugins/common.py: https://github.com/certbot/certbot/blob/main/certbot/certbot/plugins/common.py#L45
 
 
 Authenticators
@@ -389,7 +389,7 @@ identifier ``metadata-1``.
 
 The script used to generate the snapcraft.yaml files for our own externally
 snapped plugins can be found at
-https://github.com/certbot/certbot/blob/master/tools/snap/generate_dnsplugins_snapcraft.sh.
+https://github.com/certbot/certbot/blob/main/tools/snap/generate_dnsplugins_snapcraft.sh.
 
 For more information on building externally snapped plugins, see the section on
 :ref:`Building snaps`.
@@ -562,7 +562,7 @@ Building the Certbot and DNS plugin snaps
 
 Instructions for how to manually build and run the Certbot snap and the externally
 snapped DNS plugins that the Certbot project supplies are located in the README
-file at https://github.com/certbot/certbot/tree/master/tools/snap.
+file at https://github.com/certbot/certbot/tree/main/tools/snap.
 
 Updating the documentation
 ==========================

certbot/docs/packaging.rst

@@ -25,7 +25,7 @@ We release packages and upload them to PyPI (wheels and source tarballs).
 
 The following scripts are used in the process:
 
-- https://github.com/certbot/certbot/blob/master/tools/release.sh
+- https://github.com/certbot/certbot/blob/main/tools/release.sh
 
 We use git tags to identify releases, using `Semantic Versioning`_. For
 example: `v0.11.1`.
@@ -50,7 +50,7 @@ key servers.
 Notes for package maintainers
 =============================
 
-0. Please use our tagged releases, not ``master``!
+0. Please use our tagged releases, not ``main``!
 
 1. Do not package ``certbot-compatibility-test`` as it's only used internally.
 

certbot/docs/using.rst

@@ -99,7 +99,7 @@ Apache
 ------
 
 The Apache plugin currently `supports
-<https://github.com/certbot/certbot/blob/master/certbot-apache/certbot_apache/_internal/entrypoint.py>`_
+<https://github.com/certbot/certbot/blob/main/certbot-apache/certbot_apache/_internal/entrypoint.py>`_
 modern OSes based on Debian, Fedora, SUSE, Gentoo, CentOS and Darwin.
 This automates both obtaining *and* installing certificates on an Apache
 webserver. To specify this plugin on the command line, simply include