No notification alerts sent due to 403 Forbidden error

### Current Behavior:
Dependency-Track is not able to send slack notification (403 forbidden)

### Steps to Reproduce:
1- Start Dependency-Track in a docker container with the "DEBUG" log level and login
2- From Projects, create a project named "test"
3- Go to "Administation", go to "Notifications" then "Alerts", create a new alert called "alert1" and choose "Slack" as publisher
4- Click on the "alert1" and update it by setting your "webhook URL" and by checking all options in "Group"
6- In a separated sceen, show the logs of your "Dependency-Track" docker container
5- Go back to your "test" project and updoad a BOM file "bom.xml" containing some vulnerabilities
6- Logs are showing:
An error was encountered publishing notification to Slack
HTTP Status : 403 Forbidden

### Expected Behavior:
Dependency-Track should be able to send the notification server

### Environment:
- Dependency-Track Version: v4.4.0 (dependencytrack/bundled)
- Distribution: Docker
- BOM Format & Version: XML format
- Database Server: H2
- Browser: edge latest version

### Additional Details:
- I debugged also the slack server and it is working properly, issue is from the Dep-Track side (even a webhook that doesn't exist example abc.xyz/abc is giving error 403 also)
- I did a curl from inside the container and the slack is working properly
- I have the same "403 forbidden" issue with DefectDojo.

2022-03-08 13:25:56,580 DEBUG [DefectDojoClient] Uploading Dependency-Track findings to DefectDojo
2022-03-08 13:25:56,585 WARN [DefectDojoClient] DefectDojo Client did not receive expected response while attempting to upload Dependency-Track findings. HTTP response code: 403 - Forbidden
2022-03-08 13:25:56,585 ERROR [DefectDojoClient] An error occurred while communicating with the DefectDojo integration point
2022-03-08 13:25:56,585 ERROR [DefectDojoClient] HTTP Status : 403 Forbidden
2022-03-08 13:25:56,585 ERROR [DefectDojoClient] Request URL : https://defectdojo_server_fqdn/api/v2/import-scan/