fix(ras): update reset password email template heading and text

[chickenn00dle]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Closes https://app.asana.com/0/1207817176293825/1208629737155851/f

This PR updates the password reset heading, text, and button to be more general for both new users setting up a password for the first time, as well as for existing users who already have a password set:

How to test the changes in this Pull Request:

1. As admin, go to Newspack > Engagement > Show Advanced Settings > Transactional Email Content
2. Reset the Set a New Password template
3. Edit the template and confirm the text matches the screenshot above
4. Send a test email and confirm the text matches again

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?