GEODE-7431: limit Benchmarks to three-at-a-time to stay within quota (#4306)

(cherry picked from commit 9ac4454614f5e25ff691cacec2ec059a04d94437)
diff --git a/ci/pipelines/geode-build/jinja.template.yml b/ci/pipelines/geode-build/jinja.template.yml
index 1d54c81..a400a4a 100644
--- a/ci/pipelines/geode-build/jinja.template.yml
+++ b/ci/pipelines/geode-build/jinja.template.yml
@@ -407,6 +407,7 @@
 
 - name: Benchmark
   public: true
+  max_in_flight: 3
   plan:
   - get: geode-ci
     passed: