Bump `numpy` lower bound to `2.3.4`

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Hey, i'm trying to use https://github.com/online-ml/river with a full ml4t stack. Online models are a good idea and API fits good with the Strategy objects :)

  × No solution found when resolving dependencies:
  ╰─▶ Because only river==0.24.2 is available and river==0.24.2 depends on numpy>=2.3.4,<3, we can
      conclude that all versions of river depend on numpy>=2.3.4,<3.
      And because ml4t-engineer==0.1.0b4 depends on numpy>=1.24.0,<2.3 and only
      ml4t-engineer<=0.1.0b4 is available, we can conclude that ml4t-engineer>=0.1.0b4 and all
      versions of river are incompatible.
      And because your project depends on ml4t-engineer>=0.1.0b4 and river, we can conclude that
      your project's requirements are unsatisfiable.

I'm vendoring ml4t for local edits. Bumping the numpy dep lower bound to 2.3.4 fixed the issue.