Removing Travis CI from GPDB GitHub repo

[Bradford Boyle]

All,

I've been reviewing the list of webhooks that are configured on the main GPDB
GitHub repo [1] and I noticed that we have a Travis CI webhook configured but
that has been failing at least as far back as 2023-06-20. The delivery response
from Travis is HTTP 503 - Service Temporarily Unavailable. Based on a
conversation with Jimmy and Karen, I think that these webhook deliveries are
failing because we have run out of credits with Travis CI.

Do we plan to add additional credits to Travis CI? If not, can we remove the
Travis CI integration and webhook from the GPDB repo? Jimmy, Karen, and I were
talking and it does seem like there is some value in the testing that the
Travis CI was doing but we're fairly confident that that testing could be
migrated from Travis to GitHub actions [2].

--Bradford

[1]: https://github.com/greenplum-db/gpdb
[2]: https://docs.github.com/en/actions/migrating-to-github-actions/manual-migrations/migrating-from-travis-ci-to-github-actions