Move test passthrough URL to semgrep-gitlab

What does this MR do?

In the recent puzzling situation of semgrep's failing integration-test CI jobs, the root cause (or the only possible explanation) turns out to be due to the changes in the external semgrep ruleset in one of the test projects -- go/custom-ruleset-synthesis. More explanation is discussed here.

This MR changes the following:

• Changes the passthrough URL to gitlab-based semgrep registry instead of gosec-based in the test project - go/custom-ruleset-synthesis
• Adds a regenerated expectation artifact for go/custom-ruleset-synthesis test project based on the changed ruleset source mentioned above.
• Includes the missing vulnerability_finding_signatures value into GITLAB_FEATURES feature-flag for using go - when using ruleset synthesis test spec to mimic the consistency across other language tests.

Does this MR meet the acceptance criteria?

• Changelog entry added NA - dev facing changes
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer