.cirrus.yml: Switch image to 14

The current default image is set to 13.4 which is going to reach EOL
soon so switch the default CI image to 14.

Will be merged into stable/14 only. This has no operational effect on
the build itself.

Approved by: emaste
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50110

(cherry picked from commit 3740606f2641e2784eca9fe75428b7de46596d3e)