Bump follow-redirects in /fixtures/concurrent/time-slicing

Bumps [follow-redirects](https://github.com/follow-redirects/follow-redirects) from 1.14.0 to 1.15.4.
- [Release notes](https://github.com/follow-redirects/follow-redirects/releases)
- [Commits](https://github.com/follow-redirects/follow-redirects/compare/v1.14.0...v1.15.4

)

---
updated-dependencies:
- dependency-name: follow-redirects
  dependency-type: indirect
...

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