Rename master -> main.

Background:
https://www.zdnet.com/article/github-to-replace-master-with-alternative-term-to-avoid-slavery-references/

.github/workflows/cross-darwin.yml

@@ -3,7 +3,7 @@ name: Darwin-Cross
 on:
   push:
     branches:
-      - master
+      - main
   pull_request:
     branches:
       - '*'

.github/workflows/cross-freebsd.yml

@@ -3,7 +3,7 @@ name: FreeBSD-Cross
 on:
   push:
     branches:
-      - master
+      - main
   pull_request:
     branches:
       - '*'

.github/workflows/cross-openbsd.yml

@@ -3,7 +3,7 @@ name: OpenBSD-Cross
 on:
   push:
     branches:
-      - master
+      - main
   pull_request:
     branches:
       - '*'

.github/workflows/cross-windows.yml

@@ -3,7 +3,7 @@ name: Windows-Cross
 on:
   push:
     branches:
-      - master
+      - main
   pull_request:
     branches:
       - '*'

.github/workflows/license.yml

@@ -3,7 +3,7 @@ name: license
 on:
   push:
     branches:
-      - master
+      - main
   pull_request:
     branches:
       - '*'

.github/workflows/linux.yml

@@ -3,7 +3,7 @@ name: Linux
 on:
   push:
     branches:
-      - master
+      - main
   pull_request:
     branches:
       - '*'

.github/workflows/staticcheck.yml

@@ -3,7 +3,7 @@ name: staticcheck
 on:
   push:
     branches:
-      - master
+      - main
   pull_request:
     branches:
       - '*'