Reduce parallelism in release environments qa

Recently, QA on release environments fails quite often with 5xx errors,
so it can be a performance issue. This commit reduce the parallel jobs
from 5 to 3 to reduce load on the instance.

Ref: https://gitlab.com/gitlab-com/gl-infra/delivery/-/issues/20374

.gitlab/ci/release-environments/main.gitlab-ci.yml

@@ -89,7 +89,7 @@ release-environments-qa:
   extends:
     - .qa-base
   timeout: 30m
-  parallel: 5
+  parallel: 3
   variables:
     QA_SCENARIO: "Test::Instance::Smoke"
     RELEASE: "${CI_REGISTRY}/${CI_PROJECT_PATH}/gitlab-ee-qa:${CI_COMMIT_SHA}"