[MNT] Set `keras` bound to fix CI

Thank you for contributing to `aeon`

I have added the following labels to this PR based on the title: [ $\color{#EC843A}{\textsf{maintenance}}$ ].

The Checks tab will show the status of our automated tests. You can click on individual test runs in the tab or "Details" in the panel below to see more information if there is a failure.

If our pre-commit code quality check fails, any trivial fixes will automatically be pushed to your PR unless it is a draft.

Don't hesitate to ask questions on the aeon Slack channel if you have any.

PR CI actions

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[ ] Run pre-commit checks for all files
[ ] Run all pytest tests and configurations
[ ] Run all notebook example tests
[ ] Run numba-disabled codecov tests
[ ] Stop automatic pre-commit fixes (always disabled for drafts)

aeon-toolkit / aeon