Upgrade go-git to v5.18.0 (#37269)

Backport of go-git upgrade to v5.18.0 for the v1.26 release branch.

Fixes GHSA-3xc5-wrhm-f963 (credential exposure on HTTP redirects).

---
This PR was written with the help of Claude Opus 4.6

Co-authored-by: Claude (Opus 4.6) <noreply@anthropic.com>

go.mod

@@ -52,7 +52,7 @@ require (
 	github.com/go-co-op/gocron/v2 v2.19.1
 	github.com/go-enry/go-enry/v2 v2.9.5
 	github.com/go-git/go-billy/v5 v5.8.0
-	github.com/go-git/go-git/v5 v5.17.2
+	github.com/go-git/go-git/v5 v5.18.0
 	github.com/go-ldap/ldap/v3 v3.4.13
 	github.com/go-redsync/redsync/v4 v4.16.0
 	github.com/go-sql-driver/mysql v1.9.3