fix: upgrade @umbraco-cms/backoffice from 15.3.1 to 15.4.0

Snyk has created this PR to upgrade @umbraco-cms/backoffice from 15.3.1 to 15.4.0.

See this package in npm:
@umbraco-cms/backoffice

See this project in Snyk:
https://app.snyk.io/org/jcdcdev/project/3d9804d5-b21c-49c2-9d46-c76f52c51ac0?utm_source=github&utm_medium=referral&page=upgrade-pr

Author: snyk-bot