EXP-5874 Add strings for Rollouts toggle and update Studies strings

[gruberb]

Add Strings for v148, as a base for the split of studies toggle in https://github.com/mozilla-mobile/firefox-ios/pull/30988

:scroll: Tickets

Jira ticket Bugzilla issue

:bulb: Description

See https://github.com/mozilla-mobile/firefox-ios/pull/30988 for full changes.

:pencil: Checklist

• [ ] I filled in the ticket numbers and a description of my work
• [ ] I updated the PR name to follow our PR naming guidelines
• [ ] I ensured unit tests pass and wrote tests for new code
• [ ] If working on UI, I checked and implemented accessibility (Dynamic Text and VoiceOver)
• [ ] If adding telemetry, I read the data stewardship requirements and will request a data review
• [ ] If adding or modifying strings, I read the guidelines and will request a string review from l10n
• [ ] If needed, I updated documentation and added comments to complex code