Upgrade jQuery

[adamchainz]

Currently, we use jQuery 1.11:

djangoproject.com/djangoproject/static/js/lib/jquery.min.js

Line 1 in aecd080

/*! jQuery v1.11.2 | (c) 2005, 2014 jQuery Foundation, Inc. | jquery.org/license */

The latest version listed on jQuery.com is 3.7.1, and jQuery 4 is in beta.

Despite the major version updates, the core API remains mostly unchanged. I think it is feasible to upgrade it for the various browser support and security improvements.

The upgrade guide should be consulted when doing so, and all pages using jQuery tested.

[adamzap]

Hello! We decided to remove jQuery instead of upgrading it in #1827.

I'm rewriting the site's JavaScript one PR at a time. You can see a lot of them here.

[adamzap]

Let me know if you think this should be reopened.