Upgrading to jQuery 3.5.1

Hi guys

I've made some progress towards upgrading to jQuery 3.5.1 (
https://issues.apache.org/jira/browse/TAP5-2734). While this is not the 
latest version (3.6.0, actually), 3.5.1 is what the latest Bootstrap 4 
release (4.6.1) depends on (see (
https://issues.apache.org/jira/browse/TAP5-2733).

jQuery 3.0+ comes with the jQuery Migrate Plugin which simplifies 
migration of older code to version 3.0+ by restoring the APIs that were 
removed. It ships as a minified production build and a non-minified 
development build. The development build additionally shows warnings in 
the browser console when removed or deprecated APIs are used.

Here is what I've done so far (in the TAP5-2734 branch):

1. overwrote jQuery 1.12.1 sources with jQuery 3.5.1 sources;
2. added the non-minified jQuery Migrate Plugin as a JS module (only 
when Tapestry is running in development mode).
3. added a test case to verify the  jquery-migrate module is loaded and 
works as advertised.

Keen to get your opinions if the jquery-migrate module should also be 
available in production mode?

Cheers

Volker