Update minimum Dask dependency versions

[tomwhite]

• Fixes #xxxx
• Tests added
• User visible changes (including notable bug fixes) are documented in changelog.rst
• New functions are listed in api.rst

[tomwhite]

Restricting to pandas < 2 fixes the problem on Windows, but I'm not sure what the underlying problem is. It would be nice to know what that is to see if it could be fixed in a less restrictive way.

[codecov-commenter]

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[tomwhite]

I haven't been able to diagnose the underlying problem, so I tried to only add the pandas < 2 restriction to Windows with the following in requirements.txt:

pandas; platform_system != "Windows"
pandas < 2; platform_system == "Windows"

This is valid for Pip, but not Conda, which we use for (part of) the Windows installation, since msprime wheels are not available on Windows.

I'm not sure how to make progress with this.

[tomwhite]

BTW this PR is to fix #1109

[tomwhite]

Closed in favour of #1122