Merge pull request #10156 from nextcloud/bump-master-3.21

Bump master to 3.21.0

app/build.gradle

@@ -56,7 +56,7 @@ configurations.configureEach {
 
 // semantic versioning for version code
 def versionMajor = 3
-def versionMinor = 20
+def versionMinor = 21
 def versionPatch = 0
 def versionBuild = 0 // 0-50=Alpha / 51-98=RC / 90-99=stable