UnhandledRejection errors should have `level:fatal` by default

[romtsn]

Description

Because we terminate the process in the case of facing an unhandled promise rejection error, we should set its level to fatal (currently it defaults to error). Note that there are multiple modes, so we might wanna do this only when the mode is strict.

[github-actions]

A PR closing this issue has just been released 🚀

This issue was referenced by PR #15720, which was included in the 9.7.0 release.