[CI] 7.x should use 7.x-SNAPSHOT

Author: picandocodigo

.ci/test-matrix.yml

@@ -1,6 +1,6 @@
 ---
 ELASTICSEARCH_VERSION:
-- 7.6-SNAPSHOT
+- 7.x-SNAPSHOT
 
 RUBY_TEST_VERSION:
 - 2.7.0

.github/workflows/7.x.yml

@@ -25,7 +25,7 @@ jobs:
         sudo sysctl -w vm.max_map_count=262144
     - uses: elastic/elastic-github-actions/elasticsearch@master
       with:
-        stack-version: 7.6-SNAPSHOT
+        stack-version: 7.x-SNAPSHOT
     - uses: ruby/setup-ruby@v1
       with:
         ruby-version: ${{ matrix.ruby }}