devel/capnproto: Update to 1.4.0

Changelog:
https://github.com/capnproto/capnproto/compare/v1.3.0...v1.4.0

Some tests are reported to fail on FreeBSD 15
(see https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=294259).
These failures do not seem to be new for 1.4.0. That is, both
1.3.0 and 1.4.0 pass on FreeBSD <=14 and both
1.3.0 and 1.4.0 have failing tests on FreeBSD >=15.

Thus, I see no reason to hold this update.

PR: 294259
Reported by: diizzy@
Approved by: maintainer timeout, 2+ weeks