Add Gerrit 3.12.6

Change-Id: Id5f4e32efe35ee1d6e0109707cc0bf9df93def21

gerrit@3.12.6.rb

diff --git a/gerrit@3.12.6.rb b/gerrit@3.12.6.rb
new file mode 100644
index 0000000..81848a1
--- /dev/null
+++ b/gerrit@3.12.6.rb
@@ -0,0 +1,17 @@
+# typed: false
+# frozen_string_literal: true
+
+require_relative "./gerritbase"
+
+class GerritAT3126 < GerritBase
+  def self.java_version
+    "21"
+  end
+
+  set_common_dependencies
+
+  version "3.12.6"
+  sha256 "7ead27593a3e74af5b9a364b3d3640f1253fe3ec2d6296a4f8d9d62a9373302b"
+  url generate_url(version)
+
+end