selinux: Permit using systemd-userdbd

The systemd-userdbd service was added in systemd 245, which was
released in March 2020 and is available in RHEL 9.  Therefore, it's
safe to assume that the systemd_userdbd_stream_connect() SELinux
interface is also available on all relevant operating systems, unless
there's reason to believe otherwise.

https://bugzilla.redhat.com/show_bug.cgi?id=2071217
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