How do I use an older jQuery version?
I had the same issue with some older Google tags plugins that did not work with jquery 3.0.  You basically have to use the D8 library override methods in your theme to do this.

This actually goes into the theme info.yml file.  You need to have the other version of jquery defined in your library.yml file as well.  Here's some excerpts from my theme files.

my_theme.info.yml

libraries-override:
  #downgrade jquery / force jquery into header
  core/jquery: my_theme/jquery


The core/jquery was found by searching thru the core to find the jquery library definition.  

my_theme.libraries.yml

jquery:
  header: true
  remote: https://github.com/jquery/jquery
  version: "2.2.4"
  license:
    name: MIT
    url: https://github.com/jquery/jquery/blob/2.2.4/LICENSE.txt
    gpl-compatible: true
  js:
    js/jquery-2.2.4.min.js: { minified: true, weight: -20 }


Note: The header: true loads jquery in the header and not the footer.  Helps a bit with embedded jQuery code that complains that jquery does not exist.  I have a local copy of jquery in the js directory of my theme as well.

This technique can override any CSS or javascript included by any module or core.