fix(mps-sync-plugin): drop support for MPS 2020.3

`SNode.setReference(SReferenceLink, SNodeReference target)` was introduced in MPS 2021.1.
Not worth maintaining the compatibility.

Author: slisson

.github/workflows/mps-compatibility.yaml

@@ -18,7 +18,6 @@ jobs:
       fail-fast: false
       matrix:
         version:
-          - "2020.3"
           - "2021.1"
           - "2021.2"
           - "2021.3"

mps-sync-plugin3/build.gradle.kts

@@ -62,7 +62,7 @@ dependencies {
 
 tasks {
     patchPluginXml {
-        sinceBuild.set("203")
+        sinceBuild.set("211")
         untilBuild.set("241.*")
     }