roadmap: Note future HA improvement

This will be brought in through a JGit update that includes
https://git.eclipse.org/r/c/jgit/jgit/+/122288 but that series needs
significant rework to account for changes on the JGit tip.

Change-Id: I6dff62739a2faa57695fef45f2ec97bd4e1feb00
diff --git a/pages/site/roadmap.md b/pages/site/roadmap.md
index cce6fec..a844152 100644
--- a/pages/site/roadmap.md
+++ b/pages/site/roadmap.md
@@ -73,3 +73,4 @@
 * Rework event infrastructure
 * Pushing of server events into the frontend
 * Protobuf for REST API entities
+* Support for race-free zero-downtime pruning of git packed objects