Update install templates to use jquery3

jQuery has known vulnerabilities in versions prior to `3.5.0`. This
commit updates the load configuration to require `jquery3` instead of
`jquery`. This will resolve the `jquery` version from `1.12.4` to
`3.5.1`, thereby alleviating the vulnerability for new installations.

backend/spec/javascripts/spec_helper.js

@@ -4,7 +4,7 @@
 //= require support/chai-jq-0.0.7
 
 //= require_self
-//= require jquery
+//= require jquery3
 //= require rails-ujs
 //= require spree/backend
 //= require_tree ./support

core/lib/generators/solidus/install/templates/vendor/assets/javascripts/spree/backend/all.js

@@ -4,7 +4,7 @@
 // It's not advisable to add code directly here, but if you do, it'll appear at the bottom of the
 // the compiled file.
 //
-//= require jquery
+//= require jquery3
 //= require rails-ujs
 //= require spree/backend
 //= require_tree .

core/lib/generators/solidus/install/templates/vendor/assets/javascripts/spree/frontend/all.js

@@ -4,7 +4,7 @@
 // It's not advisable to add code directly here, but if you do, it'll appear at the bottom of the
 // the compiled file.
 //
-//= require jquery
+//= require jquery3
 //= require rails-ujs
 //= require spree/frontend
 //= require_tree .

guides/source/developers/assets/asset-management.html.md

@@ -102,7 +102,7 @@ you that your Solidus backend include jQuery and any other files that you create
 in this `spree/backend` directory:
 
 ```javascript
-//= require jquery
+//= require jquery3
 //= require rails-ujs
 //= require spree/backend
 //= require_tree .