github: Reconfigure Renovate to group/schedule more updates

[bkeryan]

• This contribution adheres to CONTRIBUTING.md.

What does this Pull Request accomplish?

Reconfigure Renovate:

• Change file format to json5 to allow comments.
• Set the timezone to US/Central for scheduling.
• Pin GitHub Action digests.
• Update GitHub Actions on weekends and group updates into a single PR.
• Update Python packages on weekends and group updates into a single PR.
• Move lock file maintenance to early Tuesday morning. This is primarily for indirect dependencies.
• Disable automerge because we don't need it with a less frequent update schedule.
• Enable security vulnerability alerts.
• Rebase stale PRs.

Why should this Pull Request be merged?

Reduce PR spam, while continuing to show changelogs when updating direct dependencies.

What testing has been done?

• Validated config syntax with npx --yes --package renovate -- renovate-config-validator
• Tested similar changes in nitypes-python, measurement-plugin-python, etc.