Update Open Source topic metadata

[trvny]

Please confirm this pull request meets the following requirements:

• I followed the contributing guidelines: https://github.com/github/explore/blob/main/CONTRIBUTING.md.
• This change is not self-promotion.

Which change are you proposing?

• Suggesting edits to an existing topic or collection
• Curating a new topic or collection
• Something that does not neatly fit into the binary options above

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Editing an existing topic or collection

I'm suggesting these edits to an existing topic or collection:

• Image (and my file is *.png, square, dimensions 288x288, size <= 75 kB)
• Content (and my changes are in index.md)

This adds a square topic image, the corresponding logo field, and the Open Source Initiative URL while preserving the existing topic description.

[github-actions]

Maintainer triage

Topics

• open-source — 56,824 repositories — topic page