Revert "Use old.stats.jenkins.io for JSON data)"; test [jenkins]

[chris48s]

Closes #10534

[github-actions]

	Warnings
⚠️	This PR modified service code for jenkins but not its test code. That's okay so long as it's refactoring existing code.

	Messages
📖	✨ Thanks for your contribution to Shields, @chris48s!

Generated by 🚫 dangerJS against 67c76e0

[chris48s]

Hmm, so it seems the "404" response from stats.jenkins.io (e.g: https://stats.jenkins.io/plugin-installation-trend/not-a-plugin.stats.json ) is actually served with a 200 OK status code.

[chris48s]

For the moment, I've raised an issue on
jenkins-infra/helpdesk#4291
If old.stats.jenkins.io goes away before that is resolved, there's a couple of possible options:

1. Accept the "not found" message for this service is "unparseable json response" and update the tests.
2. Switch this badge to make a raw call to _request() instead of _requestJson(). If we get back a 200 OK with a HTML body, assume that is a "not found" response. Otherwise, parse it as JSON.

[PyvesB]

Slight preference for option 2.