GitHub Actions: update to checkout@v4
We seem to be getting "Node.js 16 actions are deprecated." warnings for jobs that use checkout@v3. Except for the i686 containers job that is kept at checkout@v1 [*], update to checkout@v4, which is said to use Node.js 20. [*] 6cf4d908 (ci(main): upgrade actions/checkout to v3, 2022-12-05) refers to https://github.com/actions/runner/issues/2115 and explains why container jobs are kept at checkout@v1. We may want to check the current status of the issue and move it to the same version as other jobs, but that is outside the scope of this step. Backported-from: e94dec0c1d (GitHub Actions: update to checkout@v4, 2024-02-02) Signed-off-by: Junio C Hamano <gitster@pobox.com> Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>