[MNT] Lift `keras` version bound

[MatthewMiddlehurst]

Describe the issue

We are having CI errors due to the latest keras update. See:

broken (3.4.0) https://github.com/aeon-toolkit/aeon/actions/runs/9660623120/job/26646621175 fine (3.3.3) https://github.com/aeon-toolkit/aeon/actions/runs/9652301805/job/26622147143

We want to eventually remove this and fix the underlying issue.

Suggest a potential alternative/fix

Unknown what it is in the new version that's causing these errors.

Additional context

No response