fix: use tag rather than timestamp in dataset changelog headings

[ivan-aksamentov]

Rather than using a timestamp in changelog headings, e.g. 2024-02-16T04:00:32Z, let's use version tag, which is a safe representation of a timestamp, e.g. 2024-02-16--04-00-32Z.

Side-by-side for comparison:

2024-02-16T04:00:32Z
2024-02-16--04-00-32Z

This will make the representation consistent across CHANGELOG.md, index.json, pathogen.json, git commit text andgit tags, filesystem paths, CLI display and args, etc.

Notable exception is web UI, which decides how to represent dates in on its own to make it look friendlier for less technical users.