fix: upgrade @umbraco-cms/backoffice from 15.3.1 to 15.4.2

Snyk has created this PR to upgrade @umbraco-cms/backoffice from 15.3.1 to 15.4.2.

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

See this project in Snyk:
https://app.snyk.io/org/jcdcdev/project/7e7a7b2d-f214-4daa-9669-6d65ffa7e3cf?utm_source=github&utm_medium=referral&page=upgrade-pr

Author: snyk-bot