MPP-3579: Welcome email cta button string update

[rafeerahman]

This PR fixes MPP-3579.

New feature description

String has changed to "Learn to use Relay", which is apart of the free onboarding work being done.

l10n: mozilla-l10n/fx-private-relay-l10n#163

This PR is currently blocked due to a failing test which requires the string "Learn to use Relay" to be translated into german. Once this is done, we can merge this PR.

Screenshot (if applicable)

How to test

• Go to http://127.0.0.1:8000/emails/first_time_user_test and verify that the button has the string "Learn to use Relay"

Checklist (Definition of Done)

• Product Owner accepted the User Story (demo of functionality completed) or waived the privilege.
• All acceptance criteria are met.
• Jira ticket has been updated (if needed) to match changes made during the development process.
• I've added or updated relevant docs in the docs/ directory
• Jira ticket has been updated (if needed) with suggestions for QA when this PR is deployed to stage.
• All UI revisions follow the coding standards, and use Protocol tokens where applicable (see /frontend/src/styles/tokens.scss).
• Commits in this PR are minimal and have descriptive commit messages.
• l10n changes have been submitted to the l10n repository, if any.

[rafeerahman]

@groovecoder Reran jobs, the string has been translated now. https://pontoon.mozilla.org/de/firefox-relay-website/en/misc.ftl/?search=first-time-user-email-cta-dashboard-button&string=290830

[say-yawn]

Previous test were failing due to strings not being translated but the latest l10n has the translations and the CI tests are passing. No pending translation file was added because the strings are already translated too.

LGTM