Update parent POM, switch to Renovate, enable dep automerge

- Update parent POM to 6.2122.v70b_7b_f659d72
- Switch from Dependabot to Renovate
- Enable GHA workflows for dependency automerge

Co-Authored-By: Claude <noreply@anthropic.com>

Author: jglick

.github/dependabot.yml

@@ -0,0 +1,6 @@
+{
+  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+  "extends": [
+    "github>jenkins-infra/.github:renovate-config"
+  ]
+}

.github/renovate.json

@@ -5,7 +5,7 @@
   <parent>
     <groupId>org.jenkins-ci.plugins</groupId>
     <artifactId>plugin</artifactId>
-    <version>6.2108.v08c2b_01b_cf4d</version>
+    <version>6.2122.v70b_7b_f659d72</version>
     <relativePath/>
   </parent>