[CI] Rename Jenkins job from master to main

Author: picandocodigo

.ci/jobs/elastic+elasticsearch-ruby+main.yml

@@ -0,0 +1,11 @@
+---
+- job:
+    name: elastic+elasticsearch-ruby+main
+    display-name: 'elastic / elasticsearch-ruby # main'
+    description: Testing the elasticsearch-ruby main branch.
+    parameters:
+    - string:
+        name: branch_specifier
+        default: refs/heads/main
+        description: the Git branch specifier to build (<branchName>, <tagName>,
+          <commitId>, etc.)