Subtitle missing text alignment option

[swashbuck]

Subject of the issue

In the schemas, the _menuHeader._textAlignment object is missing an option for _subtitle.

Similar to adaptlearning/adapt-contrib-vanilla#555

Steps to reproduce

1. In course.json, add a subtitle value
2. Set all three _menuHeader._textAlignment properties to "center"

Actual behaviour

All menu header text is centered except for the subtitle.

[github-actions]

🎉 This issue has been resolved in version 7.5.1 🎉

The release is available on GitHub release

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