Migrate off probot-CLA to new GitHub Action

[cursedcoder]

Hello 👋

We are deprecating probot usage, so all Shopify repos should be migrated to use new GitHub CLA Action.

You can see how this works in vscode-shopify-ruby repo.

If you have any questions, please reach us on Slack.

After you merge this PR, make sure to update branch protection settings (if you have any):

1. Go to Settings > Branches > Edit main (or any default) branch
2. Remove the CLA check from probot (click x):
3. Add CLA check from GitHub action:

** In order for the cheak to appear on the list, the workflow should be ran for at least 1 time, so after you merge this PR, add comment with text signed, then refresh the settings page.

How to trigger workflow on existing PRs:

There are few ways how to start the check on PRs created before this change:

a) Close and reopen PR:

b) Push empty commit to trigger the events: git commit --allow-empty -m "wakey wakey GitHub Actions"

[ghost]

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[ghost]

🎉 Thanks for your contribution to Polaris!