Add auto-merger workflow to periodically merge main into release branch

[stmontgomery]

This adds a GitHub Actions workflow which automatically creates a PR to merge the main branch into the configured release branch on a set schedule.

Motivation:

This matches a practice a few other Swift repos have done recently, such as Swift package manager in swiftlang/swift-package-manager#9391.

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[grynspan]

Makes sense. Other than deleting it, how do we turn it off later?

[stmontgomery]

Makes sense. Other than deleting it, how do we turn it off later?

See comment which links to instructions