Syslog decorated mapper target not present in fields drop-down menu

## Expected Behavior
Add a dashboard where a syslog severity mapper is used for a message table. The target field should be in the FIELDS with other fields (like timestamp, source...).

## Current Behavior
The mapped field (severity in my case) is not in the list (when clicked on Add a field).


## Possible Solution

## Steps to Reproduce (for bugs)

1. Create a new dashboard.
2. Create message table, edit, decorators syslog severity mapper:
source field: level
target field: severity
3. Severity is not in the list of FIELDS

## Context

## Your Environment

* Graylog Version: 5.2.7
* Java Version: 17
* OpenSearch Version: 2.14
* MongoDB Version: 7.0.9
* Operating System: Linux Gentoo
* Browser version: Chromium 125