Remove support for JSP templates from Essentials

For a few minor releases now, we included the heads-up/warning/announcement that JSP will be phased out of Essentials in version 13, without any customer/partner complaints yet.

I'm creating this issue in order not to forget this effort.

When implementing this, I expect the following changes to be required:

1) remove all jsp templates from the plugins.
2) remove the corresponding XML installation instructions (typically grouped into an <instructionSet group="jsp">)
3) remove the template type selection mechanism from the introduction page (dashboard/src/main/webapp/pages/introduction.html) as well as the "Essentials simple plugin installation" directive template (plugin-sdk/api/src/main/resources/META-INF/resources/dashboard/api/templates/essentials-simple-install-plugin.html), along with the backing AngularJS code
4) remove support for the template selection from the "generic installation parameters" of the back-end (see for example dashboard/src/main/java/org/onehippo/cms7/essentials/rest/PluginResource.java#ensureGenericInstallationParameters).
5) update documentation and screenshots
6) provide upgrade documentation (for Essentials plugin developers), describing the API changes.

I'd estimate this to be a "small" effort.