.github: rename master to main

.github/ISSUE_TEMPLATE/bug_report.md

@@ -5,7 +5,7 @@ about: Create a report to help us improve
 
 
 **What version of OR-Tools and what language are you using?**
-Version: master/v7.3/v7.2 etc.
+Version: main/v9.4/v9.3 etc.
 Language: C++/Java/Python/C#
 
 **Which solver are you using (e.g. CP-SAT, Routing Solver, GLOP, BOP, Gurobi)**
@@ -25,4 +25,4 @@ Steps to reproduce the behavior:
 
 Make sure you include information that can help us debug (full error message, model Proto).
 
-**Anything else we should know about your project / environment**
+**Anything else we should know about your project / environment**

.github/pull_request_template.md

@@ -1,7 +1,7 @@
 <!--
 Thank you for submitting a PR!
 
-Please make sure you are targeting the master branch instead of stable and that all contributors have signed the Contributor License Agreement.
+Please make sure you are targeting the main branch instead of stable and that all contributors have signed the Contributor License Agreement.
 
 This simply gives us permission to use and redistribute your contributions as part of the project.
 Head over to https://cla.developers.google.com/ to see your current agreements on file or to sign a new one.
@@ -9,4 +9,4 @@ Head over to https://cla.developers.google.com/ to see your current agreements o
 This project follows https://opensource.google.com/conduct/
 
 Thanks!
--->
+-->

.github/workflows/check_format.yml

@@ -8,8 +8,8 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2
-    - name: Fetch origin/master
-      run: git fetch origin master
+    - name: Fetch origin/main
+      run: git fetch origin main
     - name: List of changed file(s)
       run: git diff --name-only FETCH_HEAD