Default value for Line Separator is missing when creating a 'DSV File from HTTP' data adapter

Default value for Line Separator is missing when creating a 'DSV File from HTTP' data adapter


## Expected Behavior

[Default value](https://github.com/Graylog2/graylog2-server/blob/723d38c259422d660e6b9f614391d7bcfacd14e3/graylog2-server/src/main/java/org/graylog2/lookup/adapters/DSVHTTPDataAdapter.java#L193) is populated in frontend.

## Current Behavior

[Default value](https://github.com/Graylog2/graylog2-server/blob/723d38c259422d660e6b9f614391d7bcfacd14e3/graylog2-server/src/main/java/org/graylog2/lookup/adapters/DSVHTTPDataAdapter.java#L193) is not populated

## Possible Solution

possible that [default value](https://github.com/Graylog2/graylog2-server/blob/723d38c259422d660e6b9f614391d7bcfacd14e3/graylog2-server/src/main/java/org/graylog2/lookup/adapters/DSVHTTPDataAdapter.java#L193) needs to be changed from `\n` to `\\n`

## Steps to Reproduce (for bugs)

1. Go to System / Lookup Tables
2. Go to Data Adapter
3. Create data adapter
4. Choose 'DSV File from HTTP'

## Context

Working on public documentation for Graylog Data Adapters,

## Your Environment

* Graylog Version: 6.1.1
* Browser version: Google Chrome Version 130.0.6723.92 (Official Build) (arm64)

Please let me know if there are any questions. I don't think think this is a recent bug as the code appears unchanged for many years.