Improve interface of RunnerUpgradeCheck

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR addresses #367848 (closed) by doing the following:

• Improve the internal implementation of Gitlab::Ci::RunnerUpgradeCheck to generate multiple suggestions keyed by version instead of status. This makes it so there can't be 2 distinct status for the same version, e.g. 14.0.1 can't be suggested both as :recommended and :available

• Return the top suggestion as an array of two elements, making for easier consumption by users of the class:

  _, new_status = upgrade_check.check_runner_upgrade_suggestion(runner_version.version)

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How to set up and validate locally

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No functional changes are expected from this refactor. There are integration tests both for Ci::Runners::ReconcileExistingRunnerVersionsService and EE::Types::Ci::RunnerType#upgrade_status that ensure that the usage is not broken.

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