Fix setindex deprecation warnings in LU decomposition

[ChrisRackauckas]

Summary

This PR fixes deprecation warnings in the LU decomposition code that appear in Julia 1.11+ CI builds. The warnings are caused by deprecated setindex(array, value, index) function calls on static vectors.

Changes Made

• src/ensemblegpukernel/linalg/lu.jl: Replace deprecated setindex(ps, 1, kp - 1) calls with modern array reconstruction
• Use [ps...] to convert SVector to Array for modification
• Reconstruct SVector after modification to maintain immutability

Problem Addressed

The code was using setindex(ps, 1, kp - 1) which is deprecated in Julia 1.11+. This was causing "lots of little indexing" deprecation warnings in the Buildkite CI as mentioned in the issue.

Solution

Replace the deprecated pattern:

ps = setindex(ps, 1, kp - 1)

With the modern approach:

# Swap elements: put 1 at position kp-1, and kp at the first position
ps_array = [ps...]
ps_array[kp - 1] = 1
ps = SVector{length(ps)}(ps_array)

Testing

These changes maintain identical functionality while using non-deprecated APIs. The existing test suite should continue to pass without deprecation warnings.

References

• Related to Buildkite CI build failures with deprecation warnings
• Addresses Julia 1.11+ compatibility issues in GPU kernelized LU decomposition

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