Application Link Status page fails to load due to Case-sensitive checks for Headers

Issue Summary

Note: this is a clone of JRA-70602, created to track the same bug that manifests across all products.

Trying to configure a new application link or loading the Application link page to list the status of existing Applinks will show "NETWORK ERROR" in red.
The REST client atlassian-rest-module:3.2.16 while reading headers (eg. Content-Type) does a case-sensitive lookup for them (headers name should be case-insensitive). So when headers are in lower case it can't parse them properly. The problem is triggered by the proxy server, which is converting all HTTP header names to lower-case.

Steps to Reproduce

1. Setup two Atlassian applications behind a proxy
2. Setup HAProxy with HTX enabled on one of the application
3. Configure Application link between the two applications

Expected Results

The application link can be configured and established

Actual Results

The application link is not configured correctly with "NETWORK ERROR" due to wrong headers.

Workaround

On the proxy side, disable the handling of headers to all lowercase:

• Example HAProxy, set no option http-use-htx
• Example for Envoy - header_casing