Extend artifact lifetime for release-related jobs

Artifacts of the "stress" jobs in GitLab CI are used for generating the QA summary for a given set of releases. It happened in the past that these artifacts were purged before the QA summary was prepared, unnecessarily prolonging the release process. A complete set of artifacts from all such jobs for a single pipeline should be less than 1 GB in size, so extend their lifetime from one day to one week as a more reasonable compromise between availability and disk space usage. (Note: these jobs are also run in scheduled pipelines, but that is expected to be acceptable in the context of artifact lifetime.)

Artifacts of the "release" jobs in GitLab CI are what we eventually publish on the FTP server. It happened in the past that these artifacts were purged before we copied them to their destination, unnecessarily prolonging the release process, even though we usually press the "Keep" button for the "release" jobs manually to keep them around indefinitely (as they are the source of what we publish in other places). Since there is only one "release" job per tag pipeline and its artifacts only take up tens of megabytes of space, keep the artifacts of the "release" jobs around indefinitely by default.

Reviewed elsewhere.