Improve wording for "URL is not whitelisted"

## What?

Improve wording for "URL is not whitelisted"

## Why?

If the user who is configuring a URL, for example, when creating a new Event Notification, is a Graylog Admin, they receive an error that the URL is not whitelisted. They are also provided a button to "Add to URL Whitelist". This workflow is clear and intuitive.

However, if the user is NOT an admin, and/or does not have the needed role to configure the URL Whitelist, they are only met with "URL is not whitelisted". This is not intuitive and not clear on what to do next. This will be common for companies who have separate Graylog users and Graylog admins.

Recommending to add language that says something to the effect of

> URL is not whitelisted. Contact your Graylog administrator to have them add this URL to the URL Whitelist

Ideally "URL Whitelist" can be hyperlinked to https://go2docs.graylog.org/current/setting_up_graylog/url_allowlist.html

## Your Environment

* Graylog Version: 6.1.2
* OpenSearch Version: 2.15.0
* MongoDB Version: 7.0.14
* Operating System: Ubuntu Server 22.04 LTS
* Browser version: Google Chrome Version 131.0.6778.85 (Official Build) (arm64)