This is admittedly the much abridged version, but...
Due to changes at Travis-CI that lead to prohibitively slow build/wait
times, we (edx) decided to switch CI/validation provider to Github
Actions [1].

While we had originally intended to keep the integration available,
following Travis-CI's drop in available open-source credits, it no
longer appears to be a viable option for the community.

As such, and given our already present vendor lock-in at Github,
we recommend using Github Actions for CI/validation in this repo.

However, for those not hosting in the Github ecosystem, you can
configure your testrunner to run `make validate.ci`, for the same
effect; the Github Action is just a wrapper for this call.

- [0] Fixes: TNL-7784
- [1] #276