net/k6: Update to 0.29.0

Feature-packed release with tons of much-requested
changes and additions [1]

[1] https://github.com/loadimpact/k6/releases/v0.29.0

The port now uses go:modules.

PR: 251499
Submitted by: Lewis Cook <vulcan@wired.sh> (maintainer)
Approved by: arrowd (mentor)
Differential Revision: https://reviews.freebsd.org/D27433