chore(monorepo-gerrit): Increase Gerrit's heap limit to 4g

(this translates to -Xmx)

Change-Id: I31bbbd247952fa6a592cb66ad144025af640d2db
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1021
Tested-by: BuildkiteCI
Reviewed-by: isomer <isomer@tvl.fyi>

ops/nixos/monorepo-gerrit.nix

@@ -23,6 +23,8 @@ in {
 
     package = config.depot.third_party.gerrit;
 
+    jvmHeapLimit = "4g";
+
     settings = {
       core.packedGitLimit = "100m";
       log.jsonLogging = true;