Readme: replace header image for dark mode

- Fix text contrast for dark mode and dimmed mode with white outlines around the black text

fixes ipfs#8024

README.md

@@ -1,6 +1,6 @@
 # go-ipfs
 
-![banner](https://ipfs.io/ipfs/QmVk7srrwahXLNmcDYvyUEJptyoxpndnRa57YJ11L4jV26/ipfs.go.png)
+![banner](https://ipfs.io/ipfs/bafykbzacecaesuqmivkauix25v6i6xxxsvsrtxknhgb5zak3xxsg2nb4dhs2u/ipfs.go.png)
 
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 [![Matrix](https://img.shields.io/badge/matrix-%23ipfs%3Amatrix.org-blue.svg?style=flat-square)](https://matrix.to/#/room/#ipfs:matrix.org)