chore: bump version to 2.0.21

update changelog to 2.0.21

Log: update changelog to 2.0.21

Author: mhduiy

debian/changelog

@@ -1,3 +1,9 @@
+dde-session (2.0.21) unstable; urgency=medium
+
+  * fix: support showing lock screen triggered by logind lock signal
+
+ -- zhangkun <zhangkun2@uniontech.com>  Wed, 18 Mar 2026 14:15:34 +0800
+
 dde-session (2.0.20) unstable; urgency=medium
 
   * fix: prevent whitebox keyring from blocking xrdp remote login