Prepare to release version v26.9.1

[github-actions]

Update the version number for releasing the latest version of Quark.
In this version, the following changes will be included.

• Add Quark malware analysis report for Chrysaor #968
• Add images and new malware entry to README #965
• docs: unify malware family report headings and parent doc title #950
• Add Quark malware analysis report for Skygofree #963
• Fix Read the Docs build by replacing images that serve WebP under a .png name #962
• Add Quark malware analysis report for EventBot #961