Hide phone number and explanation if homeserver doesn't have 3pid capability enabled

[hughns]

This should have been included in https://github.com/vector-im/element-ios/issues/7645 but was forgotten.

Pull Request Checklist

• [ ] I read the contributing guide
• [ ] UI change has been tested on both light and dark themes, in portrait and landscape orientations and on iPhone and iPad simulators
• [ ] Accessibility has been taken into account.
• [x] Pull request is based on the develop branch
• [x] Pull request contains a changelog file in ./changelog.d
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[sonarcloud[bot]]

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