Replace Moment.js to an alternative

[toru-takahashi]

MomentJS is used in Digdag project tightly when we organize timestamp data. But, Unfortunately, Moment.js announced that they are going to be a legacy project in maintenance mode. We need to consider replacing Moment.js to an alternative.

https://momentjs.com/docs/#/-project-status/

Immediate actions are not required but we need to investigate a potential alternative library.