What does this PR change? What problem does it solve?

The timestamps were already stored in the created snapshot. Thus also add them to the json output of the backup command for consistency.

Was the change previously discussed in an issue or on the forum?

https://forum.restic.net/t/understanding-snapshot-summaries-and-stats-ouput/8467/3

Checklist

• I have read the contribution guidelines.
• I have enabled maintainer edits.
• I have added tests for all code changes.
• I have added documentation for relevant changes (in the manual).
• There's a new file in changelog/unreleased/ that describes the changes for our users (see template).
• I have run gofmt on the code in all commits.
• All commit messages are formatted in the same style as the other commits in the repo.
• I'm done! This pull request is ready for review.