feat(gerrit-update): Add helper script for updating Gerrit schema

I've been running a script similar to this after doing Gerrit version
bumps to make sure the schema is up to date, but in the spirit of making
sure someone other that myself can do this task I'm formalising it into
the depot, where I should've put it in the first place.

Change-Id: I50a198e798e2ff26989b01e4bdd0571d85ab62aa
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2203
Tested-by: BuildkiteCI
Reviewed-by: glittershark <grfn@gws.fyi>

bin/__dispatch.sh

@@ -28,6 +28,9 @@ case "${TARGET_TOOL}" in
   gerrit)
     attr="tools.gerrit-cli"
     ;;
+  gerrit-update)
+    attr="tools.gerrit-update"
+    ;;
   hash-password)
     attr="tools.hash-password"
     ;;