Update Kokkos to release candidate

[Rohit-Kakodkar]

Description

Please describe the changes/features in this pull request.

No regression on nightly benchmarks.

Issue Number

If there is an issue created for these changes, link it here

Checklist

Please make sure to check developer documentation on specfem docs.

• I ran the code through pre-commit to check style
• THE DOCUMENTATION BUILDS WITHOUT WARNINGS/ERRORS
• I have added labels to the PR (see right hand side of the PR page)
• My code passes all the integration tests
• I have added sufficient unittests to test my changes
• I have added/updated documentation for the changes I am proposing
• I have updated CMakeLists to ensure my code builds
• My code builds across all platforms

[Copilot]

Pull request overview

Updates the CMake-based FetchContent configuration for the Kokkos dependency so the project pulls from the Kokkos 5.2.0 release-candidate ref rather than a specific commit SHA.

Changes:

• Switch Kokkos FetchContent ref from a pinned commit SHA to release-candidate-5.2.0.
• Update the inline comment describing the Kokkos ref being used.

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