Got 503 while testing connection on configuration tab

### Jenkins-Gitlab connectivity

While I trying to configure integration with my gitlab server and test it I get error 503. And it's weird, but there are no traffic I see with tcpdump on ports 80 or 443. So it seems there is some bug here and configuration tab functionality just not working. Meanwhile gitlab is accessed successfully by ping and curl from Jenkins host

Jenkins Version 2.401.2
GitLab Plugin 1.7.15

### What Operating System are you using (both controller, and any agents involved in the problem)?

Ubuntu 22.04 LTS

### Reproduction steps

1. Install Gitlab plugin
2. Configure Gitlab plugin

### Expected Results

Test OK

### Actual Results

503 Service unavailable

### Anything else?

_No response_