Blocks: Fix compatibility issues with WordPress 5.6/Gutenberg 9.5

[ryelle]

I was testing Gutenberg 9.5 on wordcamp.org earlier today and found some date & api bugs.

• Dates: Moment had a wider range of permitted date formats, but now that wp.date uses date-fns, these unsupported dates fail and crash the block. Seen on the frontend, a page with the schedule block.
• API: At some point, the requests made by the schedule block stopped being understood by getEntities/getEntityRecords, and now always return null. Seen in the editor, the schedule block never stops "loading" even though the requests finish.

The fix for the first is straightforward, I don't think it has any unexpected side-effects. The second issue is "fixed" for now by removing _fields, in the interest of getting something working. I'll continue investigating the root issue, but this could be merged if we need a quick fix.

How to test the changes in this Pull Request:

Update gutenberg to 9.5 (for the date issue). The API issue is visible on production currently (WP 5.6).

1. Have sessions on your site
2. Add a schedule block to a page
3. View the block in the editor, it should work.
4. Publish the page, the schedule should appear correct on the frontend
5. The dates & times should appear correct

[iandunn]

Moment had a wider range of permitted date formats, but now that wp.date uses date-fns, these unsupported dates fail and crash the block

That seems like a back-compat break w/ wp.date to me. What do you think about fixing that upstream instead of here?

[ryelle]

Looks like the date issue will be fixed in GB 9.6, since they've reverted the moment/date-fns switch: WordPress/gutenberg#27398

I spent some time Friday trying to figure out what exactly the fields issue is- turns out it is a core bug, so I've got a PR for that up here: WordPress/gutenberg#27526 — this workaround can go in for now, and if/when that's merged we can bring back the fields subset.

I'm going to merge this so we can jump up to 9.5, and since the _fields issue is happening on production already. When 9.6 comes out, I'll check dates again and update.