Update branches names. (jupyter#750)

Author: Carreau

.github/workflows/validate.yml

@@ -2,9 +2,9 @@ name: Test
 
 on:
   push:
-    branches: [ master ]
+    branches: [ master, main ]
   pull_request:
-    branches: [ master ]
+    branches: [ master, main ]
   workflow_dispatch:
 
 concurrency:

community.md

@@ -93,7 +93,7 @@ Jupyter also has a number of online communication channels to help keep in touch
 
 As you interact with others in the Jupyter ecosystem, remember that we have a strong
 commitment to being an **open, inclusive, and positive community**. Please read the
-[Jupyter Code of Conduct](https://github.com/jupyter/governance/blob/master/conduct/code_of_conduct.md)
+[Jupyter Code of Conduct](https://github.com/jupyter/governance/blob/main/conduct/code_of_conduct.md)
 for guidance on how to interact with others in a way that makes the community thrive.
 
 Below is a short list of Gitter channels, mailing lists, and GitHub repositories