Update to the new main default branch

Author: jtpio

.github/workflows/test.yml

@@ -1,7 +1,7 @@
 name: Tests
 on:
   push:
-    branches: ["master"]
+    branches: ["main"]
   pull_request:
     branches: ["*"]
 jobs:

docs/source/conf.py

@@ -75,7 +75,7 @@
 html_context = {
     "github_user": "jupyterlab",  # Username
     "github_repo": "jupyterlab_server",  # Repo name
-    "github_version": "master",  # Version
+    "github_version": "main",  # Version
     "doc_path": "/docs/source/",  # Path in the checkout to the docs root
 }