Maintenance: Align ES testing strategy with other service containers

(cherry picked from commit 654316a39a0e293bef042871c235f53e1c3826dd)

Co-authored-by: Martin Gruner <mg@zammad.com>

.gitlab/ci/__includes__/variables.yml

@@ -17,7 +17,7 @@ variables:
 
   # Settings for elasticsearch, if active
   ES_INDEX: 'estest.ci'
-  ELASTICSEARCH_TAG: '7.16'
+  ELASTICSEARCH_TAG: 'stable'
 
   # Define image versions. Regular commits test against 'stable' (lowest supported version),
   #   scheduled pipelines against 'latest' (newest stable release of the services).

.gitlab/ci/test/integration/es.yml

@@ -13,21 +13,15 @@
     - bundle exec rspec --tag searchindex --tag ~type:system --profile 10
 
 # Lowest supported version
-rspec:es:7.8:
+rspec:es:stable:
   extends:
     - .template_integration_es
   variables:
-    ELASTICSEARCH_TAG: '7.8.1'
+    ELASTICSEARCH_TAG: 'stable'
 
 # Highest supported version (except for newer releases that are not yet in CI)
-rspec:es:7.16:
+rspec:es:latest:
   extends:
     - .template_integration_es
   variables:
-    ELASTICSEARCH_TAG: '7.16'
-
-rspec:es:8.1:
-  extends:
-    - .template_integration_es
-  variables:
-    ELASTICSEARCH_TAG: '8.1'
+    ELASTICSEARCH_TAG: 'latest'