.github/workflows: Switch actions to default branches

Update the Coverity and mail notification workflows to use the
current default branches of the corresponding GitHub Actions.

.github/workflows/coverity.yml

@@ -23,7 +23,7 @@ jobs:
           echo "version=$version" >> $GITHUB_OUTPUT
 
       - name: Run Coverity Build
-        uses: vapier/coverity-scan-action@v1
+        uses: vapier/coverity-scan-action@main
         with:
           # Project name in Coverity Scan.
           #

.github/workflows/mail_notification.yml

@@ -58,7 +58,7 @@ jobs:
           fi
 
       - name: Send mail
-        uses: dawidd6/action-send-mail@v3
+        uses: dawidd6/action-send-mail@master
         with:
           # Required mail server address:
           server_address: smtp.yandex.com