SubmittingPatches: replace discussion of Travis with GitHub Actions
Replace the discussion of Travis CI added in
0e5d028a7a0 (Documentation: add setup instructions for Travis CI,
2016-05-02) with something that covers the GitHub Actions added in
889cacb6897 (ci: configure GitHub Actions for CI/PR, 2020-04-11).
The setup is trivial compared to using Travis, and it even works on
Windows (that "hopefully soon" comment was probably out-of-date on
Travis as well).
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>