Update master branch to main branch as default branch (#7063)

# Pull Request

## 📖 Description

This change updates documentation and pipelines to reference `main` as default branch name instead of `master`.

## 👩‍💻 Reviewer Notes

This change also updates broken links in the versioned docs to point to the archived branch. The builds may not kick off properly for this PR however since these are only pipeline and documentation updates I expect nothing should break and future PRs will not be affected.

## ✅ Checklist

### General

<!--- Review the list and put an x in the boxes that apply. -->

- [ ] I have included a change request file using `$ npm run change`
- [ ] I have added tests for my changes.
- [x] I have tested my changes.
- [x] I have updated the project documentation to reflect my changes.
- [x] I have read the [CONTRIBUTING](https://github.com/microsoft/fast/blob/master/CONTRIBUTING.md) documentation and followed the [standards](https://github.com/microsoft/fast/blob/master/CODE_OF_CONDUCT.md#our-standards) for this project.

## ⏭ Next Steps

After this PR is merged, a new branch will be created from `master` called `main` and be set as the default branch.

Author: janechu

.github/pull_request_template.md

@@ -47,7 +47,7 @@ Please provide a summary of the tests affected by this work and any unique strat
 - [ ] I have added tests for my changes.
 - [ ] I have tested my changes.
 - [ ] I have updated the project documentation to reflect my changes.
-- [ ] I have read the [CONTRIBUTING](https://github.com/microsoft/fast/blob/master/CONTRIBUTING.md) documentation and followed the [standards](https://github.com/microsoft/fast/blob/master/CODE_OF_CONDUCT.md#our-standards) for this project.
+- [ ] I have read the [CONTRIBUTING](https://github.com/microsoft/fast/blob/main/CONTRIBUTING.md) documentation and followed the [standards](https://github.com/microsoft/fast/blob/main/CODE_OF_CONDUCT.md#our-standards) for this project.
 
 ## ⏭ Next Steps
 

.github/workflows/ci-validate-platforms.yml

@@ -4,11 +4,11 @@ on:
   workflow_dispatch:
   push:
     branches:
-      - master
+      - main
 
   pull_request:
     branches:
-      - master
+      - main
 
   schedule:
     - cron: 0 7 * * 3
@@ -37,7 +37,7 @@ jobs:
 
       - if: ${{ github.event_name == 'pull_request' }}
         run: |
-          git fetch --no-tags --prune --depth=1 origin +refs/heads/master:refs/remotes/origin/master
+          git fetch --no-tags --prune --depth=1 origin +refs/heads/main:refs/remotes/origin/main
 
       - name: Cache multiple paths
         uses: actions/cache@v2