[stable20] Add changelog for 9.0.8 and 10.0.6

[nickvergessen]

10.0.6 – 2021-02-22

🔧 Changed

• Replace blur with average color in video backgrounds to improve performance on Chrome based browsers #5012

🐞 Fixed

• Drop guest moderators when changing a conversation to group conversation #5228
• Fix collaboration resource options not loading #5142
• Fixed a bug that would prevent attachments going into the Talk/ folder #5079
• Fix mentioning of users with subnames, e.g. "foo" and "foobar" #5051
• Add upload editor in files sidebar mode #5111

[PVince81]

👍