ci: add Renovate for Easy-RSA version updates (#1333)

## Summary

- Add Renovate configuration to automatically track Easy-RSA releases
- Add GitHub Action to auto-update SHA256 hash on Renovate PRs

## How it works

1. **Renovate** detects a new Easy-RSA release → creates PR updating
`EASYRSA_VERSION`
2. **GitHub Action** triggers on the PR → downloads tarball → computes
SHA256 → commits fix
3. PR is ready to merge with both version and hash updated

---

I intentionally updated to the second-to-last version in
https://github.com/angristan/openvpn-install/commit/bda450948a933224f4f779a24a44c6279e1574a1
to test if this works.

renovate.json

@@ -0,0 +1,16 @@
+{
+  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+  "extends": ["config:recommended"],
+  "customManagers": [
+    {
+      "customType": "regex",
+      "managerFilePatterns": ["/^openvpn-install\\.sh$/"],
+      "matchStrings": [
+        "readonly\\s+EASYRSA_VERSION=\"(?<currentValue>\\d+\\.\\d+\\.\\d+)\""
+      ],
+      "depNameTemplate": "OpenVPN/easy-rsa",
+      "datasourceTemplate": "github-releases",
+      "extractVersionTemplate": "^v(?<version>.*)$"
+    }
+  ]
+}