Update part of the "Working with IndexedDB" training for new idb.

[jyasskin]

I didn't go through the whole thing exhaustively, so there are almost certainly other problems.

What's changed, or what was fixed?

• Parts used the old idb interface, pre https://github.com/jakearchibald/idb/blob/master/changes.md#changes-in-4x

Fixes: I didn't file an issue

Target Live Date: Not urgent

• This has been reviewed and approved by (NAME)
• I have run npm test locally and all tests pass.
• I have added the appropriate type-something label.
• I've staged the site and manually verified that my content displays correctly.

^ Ha, nope. I just typed in the github interface. ❤️

CC: @petele