Document pyRiemann 0.9 and numpy 2.0 compatibility update in changelog

[Copilot]

Summary

This PR adds a changelog entry to document the pyRiemann 0.9 and numpy 2.0 compatibility update that was implemented in PR #789, addressing issue #817.

Background

Issue #817 reported compatibility conflicts between moabb and pyriemann, particularly with numpy version requirements. While PR #789 successfully resolved these conflicts by updating to pyriemann 0.9 and numpy 2.0, it was marked with "no changelog" and therefore the fix was not documented in whats_new.rst.

Since users are actively asking about this compatibility issue, it's important to properly document this enhancement in the changelog so users know the fix is available in the develop branch and will be included in the next release (v1.4).

Changes

• Added changelog entry in docs/source/whats_new.rst under the "Develop branch - 1.4 (dev)" Enhancements section
• Entry properly references PR Update to pyRiemann 0.9 and mne 1.10 #789 and credits @gcattan

Current Dependencies (from PR #789)

• python = ">=3.10"
• numpy = ">=2.0"
• pyriemann = "^0.9"
• mne = "^1.10.0"

These updated dependencies ensure compatibility between moabb and pyriemann, eliminating the numpy version conflicts that users were experiencing.

Closes #817

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Compatability with Pyriemann</issue_title>
<issue_description>Hello,
When trying to use Moabb and Pyriemann, there are conflicts of requirements for packages, e.g., numpy. I realize this is not a trivial task, but to update the Moabb package to be compatible with newer versions of, e.g., numpy would make it easier to combine Moabb with other developing Python tools.

Have a nice day!

//Frida</issue_description>

Comments on the Issue (you are @copilot in this section)

@gcattan Hi Frida, this issue was addressed in https://github.com//pull/789, and the fix should be available for you in next MOABB release.

During the meantime, you can still use the devop branch.
Let me know if you experience some issue there. </comment_new>

Fixes #817

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