Rename master to main

[travisbrown]

This is a minor thing, but it's easy and will likely soon be the default on GitHub.

I've only changed links and references to the master branch in current instructions or documentation, not references in historical contexts like the changelog. I've chosen main since there seems to be consensus that that's the best replacement, but I'm happy to change it to one of the other alternatives.

There will be a few other steps that are necessary (see here or here) in conjunction with this PR, which just ensures that we don't break links or CI.

[travisbrown]

(When I call this a "minor thing" I don't mean to downplay concerns about exclusionary language, just to note that e.g. GitHub's actions on this issue don't really deserve much celebration in the context of their continued relationship with ICE.)

[codecov-commenter]

Codecov Report

Merging #3480 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3480   +/-   ##
=======================================
  Coverage   91.77%   91.77%           
=======================================
  Files         383      383           
  Lines        8394     8394           
  Branches      205      205           
=======================================
  Hits         7704     7704           
  Misses        690      690           

[mpilquist]

Thanks for doing this!

[rossabaker]

So far, I've only made this move on repos with empty PR queues. Do we just change the target branch on the open ones, or do we have to reopen them?

I'm 👍 on the move either way.

[travisbrown]

@rossabaker I made this change in Dhall for Java, and had to change the target branch for open PRs, but that's all—I didn't have to reopen them.

[djspiewak]

This appears to be overwhelmingly approved. Just staying open to track the other juggling required to make this happen.

[larsrh]

Superseded by #3833.