trusted_proxies missing for Trusted Header Authentication

## Expected Behavior

Adding proxies to graylog.conf file should be reflected in the Graylog application.

## Current Behavior

Using Docker image `graylog/graylog:4.0`. Despite adding trusted proxies to the graylog.conf file, the `Please configure the trusted_proxies setting in the Graylog server configuration file.` info text persists, and connection attempts from localhost using the trusted header are denied.

Variants tested, to make sure it's not because of whitespace (and to match the format in the [IPSubnetConverterTest](https://github.com/Graylog2/graylog2-server/blob/master/graylog2-server/src/test/java/org/graylog2/utilities/IPSubnetConverterTest.java)):
```
trusted_proxies = 127.0.0.1/32, 0:0:0:0:0:0:0:1/128
```
```
trusted_proxies = 127.0.0.1/32,0:0:0:0:0:0:0:1/128
```

This has worked in previous versions, possible regression as of the SSO-inclusion into the core functionalities in Graylog 4?

Since this is in a container environment, I've verified that the settings are present in the container and appear to be in the correct format (as listed above). These settings are loaded using a config map in Kubernetes, and should thus be read at startup by Graylog.

## Your Environment

* Graylog Version: 4.0.1+6a0cc0b (using container image)
* Java Version: 1.8.0_275
* Elasticsearch Version: 7.10.1
* MongoDB Version: 4.4.2
* Operating System: Debian GNU/Linux 10 (buster)
* Browser version: Mozilla Firefox 84.0