Replace image aria-label with alt and remove redundant aria-hidden

[kirsty-hames]

Subject of the enhancement

To align with core change of using alt instead of aria-label - adaptlearning/adapt-contrib-core#822

Testing confirmed all major screen readers handle alt and aria-label equivalently and Adapt would benefit from the broader benefits of alt. Benefits include:

• alternative text displayed when image fails to load
• alternative text retained when copying images into Word or similar tools.

Testing also confirmed an empty alt tag (alt="") is widely supported by screen readers / browsers to ignore an image without the need for aria-hidden.

Applies to image, line 4.

[github-actions]

🎉 This issue has been resolved in version 4.10.0 🎉

The release is available on GitHub release

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