doc: update v30 migration guide for jest.mock() strictly case-sentitive path

[hainenber]

Summary

Closes #15847

Update v30 migration guide on changed behavior of jest.mock() to only work with case-sensitive paths from now, unlike Jest 29.

The change behavior seems to originate at this PR, starting from 30.0.0-beta.6.

Test plan

• Green CI
• Confirmed from local reproduction.

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commit: d44f02c