Add support for multi-ecosystem updates in job processing

[markhallen]

What are you trying to accomplish?

Update jobs have a predicate method UpdateJob#multi_ecosystem_update? that can be used to determine if the job is part of a multi ecosystem update.

This PR appropriately handles multi ecosystem update jobs.

Part of #8126

Anything you want to highlight for special attention from reviewers?

We don't need to handle grouping the multi-ecosystem PRs in dependabot-core.

How will you know you've accomplished your goal?

We will test a multi ecosystem pull request with @dependabot recreate and @dependabot rebase.

Checklist

• I have run the complete test suite to ensure all tests and linters pass.
• I have thoroughly tested my code changes to ensure they work as expected, including adding additional tests for new functionality.
• I have written clear and descriptive commit messages.
• I have provided a detailed description of the changes in the pull request, including the problem it addresses, how it fixes the problem, and any relevant details about the implementation.
• I have ensured that the code is well-documented and easy to understand.