CI: Bump APP_DEV_VERSION to 0.5.1.21 [skip ci]

Config.xcconfig

@@ -19,7 +19,7 @@ TRIO_APP_GROUP_ID = group.org.nightscout.$(DEVELOPMENT_TEAM).trio.trio-app-group
 
 // The developers set the version numbers, please leave them alone
 APP_VERSION = 0.5.1
-APP_DEV_VERSION = 0.5.1.20
+APP_DEV_VERSION = 0.5.1.21
 APP_BUILD_NUMBER = 1
 COPYRIGHT_NOTICE =