Meta: default branch rename

Helps with whatwg/meta#174.

Author: annevk

.github/workflows/build.yml

@@ -2,10 +2,10 @@ name: Build
 on:
   pull_request:
     branches:
-    - master
+    - main
   push:
     branches:
-    - master
+    - main
 jobs:
   build:
     name: Build

CONTRIBUTING.md

@@ -1 +1 @@
-Please see the [WHATWG Contributor Guidelines](https://github.com/whatwg/meta/blob/master/CONTRIBUTING.md).
+Please see the [WHATWG Contributor Guidelines](https://github.com/whatwg/meta/blob/main/CONTRIBUTING.md).

PULL_REQUEST_TEMPLATE.md

@@ -7,7 +7,7 @@ Thank you for contributing to the MIME Sniffing Standard! Please describe the ch
    * …
 - [ ] [Tests](https://github.com/web-platform-tests/wpt) are written and can be reviewed and commented upon at:
    * …
-- [ ] [Implementation bugs](https://github.com/whatwg/meta/blob/master/MAINTAINERS.md#handling-pull-requests) are filed:
+- [ ] [Implementation bugs](https://github.com/whatwg/meta/blob/main/MAINTAINERS.md#handling-pull-requests) are filed:
    * Chrome: …
    * Firefox: …
    * Safari: …

README.md

@@ -16,7 +16,7 @@ started, leave a comment on the issue or ask around [on IRC](https://whatwg.org/
 ## Pull requests
 
 In short, change `mimesniff.bs` and submit your patch, with a
-[good commit message](https://github.com/whatwg/meta/blob/master/COMMITTING.md). Consider
+[good commit message](https://github.com/whatwg/meta/blob/main/COMMITTING.md). Consider
 reading through the [WHATWG FAQ](https://whatwg.org/faq) if you are new here.
 
 Please add your name to the Acknowledgments section in your first pull request, even for trivial
@@ -26,12 +26,12 @@ fixes. The names are sorted lexicographically.
 
 For quick local iteration, run `make`. To verify your changes locally, run `make deploy`. See more
 in the
-[WHATWG Contributor Guidelines](https://github.com/whatwg/meta/blob/master/CONTRIBUTING.md#building).
+[WHATWG Contributor Guidelines](https://github.com/whatwg/meta/blob/main/CONTRIBUTING.md#building).
 
 ## Merge policy
 
 If you can commit to this repository, see the
-[WHATWG Maintainer Guidelines](https://github.com/whatwg/meta/blob/master/MAINTAINERS.md).
+[WHATWG Maintainer Guidelines](https://github.com/whatwg/meta/blob/main/MAINTAINERS.md).
 
 ## Tests