Update Backbone to latest version

[robyngit]

Current version: 1.3.3 (Apr. 5, 2016)
Latest version: 1.6.0 (Feb. 5, 2024)

Backbone has had several updates in recent years. Improvements include one that will allow our collections to be compatible with our new linting rules, as well as some bug fixes. There look to be no breaking changes mentioned in the changelog. Also, it appears the author is using semantic versioning and the newer version is still part of the same major version as the one we're using. Still, we'll have to do some pretty thorough testing before officially switching.

We should keep up-to-date moving forward so that we don't have to skip a bunch of intermediate versions in case a change in one does cause a bug. It looks like the backbone repo is still under active development.

[robyngit]

(Also, the author opened an issue in May of this year asking the community what they'd like to see in Backbone 2.0!)