[!222] update dune-composites.md to releases/2.6

Merge branch 'master' into 'master'

See merge request [!222]

  [!222]: Noneinfrastructure/dune-website/merge_requests/222
4 jobs for master in 36 minutes and 14 seconds (queued for 3 seconds)
Status Job ID Name Coverage
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  Documentation
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  Deploy
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website-deploy
deploy

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