Add Gerrit 3.12.8

Change-Id: I9fa4768ffd5f9cc56530d870c98de2541949a731

gerrit@3.12.8.rb

diff --git a/gerrit@3.12.8.rb b/gerrit@3.12.8.rb
new file mode 100644
index 0000000..e734fea
--- /dev/null
+++ b/gerrit@3.12.8.rb
@@ -0,0 +1,17 @@
+# typed: false
+# frozen_string_literal: true
+
+require_relative "./gerritbase"
+
+class GerritAT3128 < GerritBase
+  def self.java_version
+    "21"
+  end
+
+  set_common_dependencies
+
+  version "3.12.8"
+  sha256 "2f7bb47eec3121731ce8c9ccf7a845b500d96c7014f48d99d01fda24f3e7c4a7"
+  url generate_url(version)
+
+end