Palo Alto Networks Firewall TCP (PAN-OS v9+) input parse error

## Expected Behavior

field **pan_source_user** contains date **2021-12-29 08:06:50.775 +00:00**

## Current Behavior

field **pan_source_user** should contain **username**

## Possible Solution

Take a look to https://docs.paloaltonetworks.com/pan-os/9-1/pan-os-admin/monitoring/use-syslog-for-monitoring/syslog-field-descriptions/user-id-log-fields.html#id23f3cbfb-946f-423f-bc48-50fdc2b68238

## Steps to Reproduce (for bugs)

1. Use Graylog 4.2.4+b643d2b on f687edde0f02 (Oracle Corporation 1.8.0_312 on Linux 5.4.0-53-generic)
2. and Palo Alto Firewall 10.0.8
3. create Palo Alto Networks Firewall TCP (PAN-OS v9+) input

## Context

## Your Environment

* Graylog Version: 4.2.4+b643d2b
* Java Version: f687edde0f02 (Oracle Corporation 1.8.0_312 on Linux 5.4.0-53-generic)
* Elasticsearch Version: 7.10.2
* MongoDB Version: 4.2
* Operating System: Ubuntu 20.04
* Browser version: FF 95