Merge pull request #5949 from nextcloud/bumpTo3.13alpha

Bump to 3.13 alpha

build.gradle

@@ -84,7 +84,7 @@ repositories {
 
 // semantic versioning for version code
 def versionMajor = 3
-def versionMinor = 12
+def versionMinor = 13
 def versionPatch = 0
 def versionBuild = 0 // 0-50=Alpha / 51-98=RC / 90-99=stable