Bump api version

Oops, forgot to do that in 76aeb72.

Author: jtojnar

src/common.php

@@ -23,7 +23,7 @@
 // independent of selfoss version
 // needs to be bumped each time public API is changed (follows semver)
 // keep in sync with docs/api-description.json
-$f3->set('apiversion', '2.20.0');
+$f3->set('apiversion', '2.21.0');
 
 $f3->set('AUTOLOAD', false);
 $f3->set('BASEDIR', BASEDIR);