Rename master branch to main (#7034)

For consistency with other repositories, i.e. so I can write `git
checkout main` without worrying which repo I'm browsing.

.github/workflows/deploy_web_demo.yml

@@ -1,9 +1,9 @@
 name: Deploy web demo
 
 on:
-  # We only run this on merges to master
+  # We only run this on merges to main
   push:
-    branches: ["master"]
+    branches: ["main"]
   # Allows you to run this workflow manually from the Actions tab
   workflow_dispatch:
 # to only run when you do a new github release, comment out above part and uncomment the below trigger.

.github/workflows/png_only_on_lfs.yml

@@ -25,7 +25,7 @@ jobs:
           exclude_pattern=$(printf "|^%s" "${exclude_paths[@]}" | sed 's/^|//')
 
           if comm -23 <(git ls-files | grep -Ev "$exclude_pattern" | sort) <(git lfs ls-files -n | sort) | grep "\.${ext}$"; then
-            echo "Error: Found binary file with extension .$ext not tracked by git LFS. See https://github.com/emilk/egui/blob/master/CONTRIBUTING.md#working-with-git-lfs"
+            echo "Error: Found binary file with extension .$ext not tracked by git LFS. See https://github.com/emilk/egui/blob/main/CONTRIBUTING.md#working-with-git-lfs"
             exit 1
           fi
         done