chore(renovate): use cron syntax for schedule (#3638)

# Summary

Change Renovate schedule to cron syntax.

Fixes #404

## Testing Process

Code inspection only

## Checklist

- [x] Review the contributing
[guidelines](https://github.com/slsa-framework/slsa-github-generator/blob/main/CONTRIBUTING.md)
- [x] Add a reference to related issues in the PR description.
- [x] Update documentation if applicable.
- [x] Add unit tests if applicable.
- [x] Add changes to the
[CHANGELOG](https://github.com/slsa-framework/slsa-github-generator/blob/main/CHANGELOG.md)
if applicable.

Signed-off-by: Rhys Arkins <rhys@arkins.net>

renovate.json

@@ -1,9 +1,9 @@
 {
   "$schema": "https://docs.renovatebot.com/renovate-schema.json",
   "extends": ["config:base", ":gitSignOff"],
-  "schedule": ["before 4 am on the first day of the month"],
+  "schedule": ["* 0-4 1 * *"],
   "vulnerabilityAlerts": {
-    "schedule": "before 4am"
+    "schedule": "* 0-4 * * *"
   },
   "postUpdateOptions": ["gomodTidy", "gomodUpdateImportPaths"],
   "packageRules": [