Update parent POM and modernize CI

- Update parent POM to 6.2122.v70b_7b_f659d72
- Switch from Dependabot to Renovate
- Enable GitHub Actions reusable workflows for CI and changelog

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

@@ -28,7 +28,7 @@
     <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>
     <groupId>org.jenkins-ci.plugins.workflow</groupId>