fix: manually set modified time to SFTP files after editing

[rikled]

Summary

This PR fixes SFTP file writes. When uploading a file to the SFTP server, the files original modified date is not preserved. The SFTP server typically sets the modified date to the time of upload. This leads to issues, for example, with Nextcloud Office, which expects the mtime to match the mtime of the file. Nextcloud office then repeatedly throws errors indicating that the file has been modified externally. In this PR the SFTP implementation explicitely sets the modified date of the file.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI