UX: Improve wording of "Data Node version mismatch" system notification

## What?

When Graylog Server is a different version than Data Node, Graylog displays the following system notificaiton:

> **Data Node version mismatch**
>
> You are running at least one Data Node which runs on a different version than your server. While this might not impose a problem, it is recommended to run server and Data Nodes on the same version.

Via https://github.com/Graylog2/graylog2-server/blob/3dd356924b9c46f4cec0d8cbdd1c2de99d4e123e/graylog2-server/src/main/resources/org/graylog2/freemarker/templates/HTML/data_node_version_mismatch.ftl#L1

the use of `server` above is ambiguous. I propose we prepend the word Graylog to server as well as "data node" to remove any ambiguity:

> You are running at least one **Graylog** Data Node which runs on a different version than your **Graylog** server

## Why?

The unqualified use of the word `server` above can be interpreted in multiple ways:

- the server graylog runs on
- the server data node runs on
- Graylog version

Let me know if there are any questions!