git/gitlab: default project webhooks to SSL verification when creating hooks

[Vi-shub]

Summary

pkg/git/gitlab/gitlab.go creates project hooks with EnableSSLVerification set to false (&f), which disables TLS certificate verification for the webhook URL.

	webhook := &gitlab.AddProjectHookOptions{
		EnableSSLVerification: &f,
		PushEvents:            &t,
		Token:                 &webhookSecret,
		URL:                   &payloadURL,
	}

Problem
For HTTPS payload URLs, disabling SSL verification is an insecure default and is inconsistent with tightening GitHub repository webhooks (pkg/git/github).

Proposal
Default EnableSSLVerification to true for HTTPS webhook targets (or always true unless an explicit opt-in exists for dev/self-signed).
This idea came as followup of #3714