[ALL] Drop Travis in favor of Apache CI and GitHub Actions

[Gary Gregory]

Hi All,

In order to ease maintenance, I propose that we drop Travis CI in favor of
Apache CI and GitHub Actions. 2 CI systems instead of 3 seems like plenty
to me. The exception would be a component with an existing complex Travis
build that was not or cannot be reproduced in GitHub Actions.

Looking ahead, I think we should consider publishing SNAPSHOT sites to
GitHub.io.

Gary