Added 8.4 job + updated tests

Author: ezimuel

.ci/jobs/elastic+elasticsearch-php+6.8.x.yml renamed to .ci/jobs/elastic+elasticsearch-php+8.4.yml

@@ -1,12 +1,12 @@
 ---
 - job:
-    name: elastic+elasticsearch-php+6.8.x
-    display-name: 'elastic / elasticsearch-php # 6.8.x'
-    description: Testing the elasticsearch-php 6.8.x branch.
+    name: elastic+elasticsearch-php+8.4
+    display-name: 'elastic / elasticsearch-php # 8.4'
+    description: Testing the elasticsearch-php 8.4 branch.
     parameters:
     - string:
         name: branch_specifier
-        default: refs/heads/6.8.x
+        default: refs/heads/8.4
         description: the Git branch specifier to build (<branchName>, <tagName>,
           <commitId>, etc.)
     triggers:

.ci/test-matrix.yml

@@ -1,6 +1,6 @@
 ---
 STACK_VERSION:
-  - 8.3.0-SNAPSHOT
+  - 8.4.0-SNAPSHOT
 
 PHP_VERSION:
   - 8.1-cli

.github/workflows/test.yml

@@ -11,7 +11,7 @@ jobs:
       matrix:
         php-version: [7.4, 8.0, 8.1]
         os: [ubuntu-latest]
-        es-version: [8.3.0-SNAPSHOT]
+        es-version: [8.4.0-SNAPSHOT]
 
     steps:
     - name: Checkout