What would you like Renovate to be able to do?

The automergeStrategy configuration item introduced in #10627 should be implemented for the GitHub platform, allowing users to configure the explicit merge strategy used by Renovate when automerging pull requests

Did you already have any implementation ideas?

The optional MergeStrategy argument can be provided to the GitHub mergePr implementation.The current implementation uses the merge strategy configured on the repository at the time it is internally constructed, with preferences based on which features are allowed. The Renovate-specific strings for automergeStrategy can be mapped to GitHub-specific strings when submitting the merge request to GitHub or aborted if the configured strategy is not listed as available.

If #10830 has been implemented, the arguments to mergePr may be different than what has been listed here. We should also remember to clean up the missing feature notice in lib/platform/github/index.md and update the description in docs/usage/configuration-options.md if necessary.