Chore: Replace `modrun` with `run`

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## 👾 Description of the issue

We had added `modrun` to the gradle build because we needed the ability both to `run` the older monolithic services code, and `modrun` to run the newer modular code. Now that we no longer support the older monolithic services code, we should remove `modrun` and configure `run` correctly to launch the modular services code. We should update the new documentation (#12813) accordingly.

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## 🤔 Additional Information

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Locate the Gradle build configuration that defines the `run` and `modrun` tasks, then review the new documentation referenced by issue #12813. Confirm that `run` launches the modular services code, remove the obsolete task, and update the documentation to match; verify the relevant Gradle task locally.

Written by the indexing model from the issue text.