Bump global-refdb to v3.4.8

The upgrade allows to have a pluggable implementation
of the underlying LocalDiskRepositoryManager, thanks to
an enhanced validation module.

The pluggability allows the LocalDiskRepositoryManager to
be wrapped by a cache layer provided by the cached-refdb
module, without having to change anything in the
high-availability.config.

Change-Id: I8bac5fad9df951fe8354e5fb6917883f9e7a197e

external_plugin_deps.bzl

diff --git a/external_plugin_deps.bzl b/external_plugin_deps.bzl
index 9af89f0..74c8f13 100644
--- a/external_plugin_deps.bzl
+++ b/external_plugin_deps.bzl
@@ -15,6 +15,6 @@
 
     maven_jar(
         name = "global-refdb",
-        artifact = "com.gerritforge:global-refdb:3.3.2.1:jdk8",
-        sha1 = "7a293d577665dfc6f4d36371af21c4a3f7177b23",
+        artifact = "com.gerritforge:global-refdb:3.4.8",
+        sha1 = "a05e1684c0b02867c203e3f55efe62ced2c0fe61",
     )