[CI] rename stable branch to main

This change updates the name of the stable branch in .travis
and .github/main.yml to main.

.github/workflows/main.yml

@@ -3,13 +3,13 @@ name: Build Tests
 on:
   push:
     branches:
-      - master
+      - main
       - develop
       - release-*
       - gh-workflows
   pull_request:
     branches:
-      - master
+      - main
       - develop
 
 env:

.travis.yml

@@ -25,7 +25,7 @@ before_install:
 
 branches:
   only:
-     - master
+     - main
      - develop
      - /^release-.*$/
      - travis-test