Update selection question type warning message for multiple branches

[lfdebrux]

What problem does this pull request solve?

Trello card: https://trello.com/c/PzJ1n9dI/3243-update-wording-of-the-banner-when-changing-whether-selection-question-can-have-more-than-one-answer

Selection questions can have one or more routing conditions when multiple branches is enabled, update the warning messages on the page for changing the type of a selection question to reflect this.

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• Ensure that you consider the wider context.
• Does it work when run on your machine?
• Is it clear what the code is doing?
• Do the commit messages explain why the changes were made?
• Are there all the unit tests needed?
• Do the end to end tests need updating before these changes will pass?
• Has all relevant documentation been updated?

[github-actions]

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