Update metainfo.xml to 1.6.8

[ci skip]

dist/linux/common/org.cryptomator.Cryptomator.metainfo.xml

@@ -66,6 +66,7 @@
 	</content_rating>
 
 	<releases>
+		<release date="2022-03-30" version="1.6.8"/>
 		<release date="2021-12-16" version="1.6.5"/>
 	</releases>
 </component>