Migrate master branch to main

We're a few years behind the curve on this one, but using "master" as a
programming term is a callous practice that explicitly uses the
historical institution of slavery as a cheap, racist metaphor. Switch to
using "main", as it's the new default in git and GitHub.

.github/workflows/merged.yaml

@@ -8,14 +8,14 @@ on:
 jobs:
   if_merged:
     # Forked repos can not access Mattermost secret.
-    if: github.event.pull_request.merged == true && !github.event.pull_request.head.repo.fork 
+    if: github.event.pull_request.merged == true && !github.event.pull_request.head.repo.fork
     runs-on: ubuntu-latest
     steps:
-    - uses: mattermost/action-mattermost-notify@master
+    - uses: mattermost/action-mattermost-notify@main
       with:
         MATTERMOST_WEBHOOK_URL: ${{ secrets.MATTERMOST_MERGE_WEBHOOK }}
         TEXT: >
           [${{ github.repository }}] |
           [${{ github.event.pull_request.title }}
           #${{ github.event.number }}](https://github.com/${{ github.repository }}/pull/${{ github.event.number }})
-          was merged into master by ${{ github.actor }}
+          was merged into main by ${{ github.actor }}

.github/workflows/notify_weekly.yaml

@@ -15,7 +15,7 @@ jobs:
         DATE=$(date --date="7 days ago" +"%Y-%m-%d")
         echo "MERGED_URL=https://github.com/pulls?q=merged%3A%3E${DATE}+org%3Acertbot" >> $GITHUB_ENV
         echo "UPDATED_URL=https://github.com/pulls?q=updated%3A%3E${DATE}+org%3Acertbot" >> $GITHUB_ENV
-    - uses: mattermost/action-mattermost-notify@master
+    - uses: mattermost/action-mattermost-notify@main
       with:
         MATTERMOST_WEBHOOK_URL: ${{ secrets.MATTERMOST_WEBHOOK_URL }}
         MATTERMOST_CHANNEL: private-certbot