Fix max_shift value in flux and related functions to 1.15 (issue #10675)

[puhuk]

What does this PR do?

This PR updates the max_shift value in flux to 1.15 for consistency across the codebase. In addition to modifying max_shift in flux, all related functions that copy and use this logic, such as calculate_shift in src/diffusers/pipelines/stable_diffusion_3/pipeline_stable_diffusion_3_img2img.py, have also been updated to ensure uniform behavior.

Fixes #10675

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@yiyixuxu and @asomoza

[HuggingFaceDocBuilderDev]

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[hlky]

Thanks @puhuk