Bump `__FreeBSD_version__` for r343891

[ngie]

Summary

This will allow upstream consumers, e.g., capsicum-test and third-party
packages (via ports(7)), to test for a specific __FreeBSD_version__ and
expect renameat(2) to be functional.

PR: 222258
MFC with: r343891

Test Plan

n/a

[emaste]

Approved