Update engine tag to release-20210321.

Author: pchote

mod.config

@@ -9,7 +9,7 @@
 MOD_ID="example"
 
 # The OpenRA engine version to use for this project.
-ENGINE_VERSION="playtest-20210131"
+ENGINE_VERSION="release-20210321"
 
 # .dll filenames compiled by the mod solution for use by the runtime assembly check
 WHITELISTED_MOD_ASSEMBLIES="OpenRA.Mods.Example.dll"