[CI] Build 7.8 on GitHub Actions, set version on matrices

Author: picandocodigo

.ci/test-matrix.yml

@@ -1,6 +1,6 @@
 ---
 ELASTICSEARCH_VERSION:
-- 7.x-SNAPSHOT
+- 7.8-SNAPSHOT
 
 RUBY_TEST_VERSION:
 - 2.7

.github/workflows/7.x.yml

@@ -3,6 +3,7 @@ on:
   push:
     branches:
       - 7.x
+      - 7.8
   pull_request:
     branches:
       - 7.x
@@ -25,7 +26,7 @@ jobs:
         sudo sysctl -w vm.max_map_count=262144
     - uses: elastic/elastic-github-actions/elasticsearch@master
       with:
-        stack-version: 7.x-SNAPSHOT
+        stack-version: 7.8-SNAPSHOT
     - uses: ruby/setup-ruby@v1
       with:
         ruby-version: ${{ matrix.ruby }}