[FLINK-37035][release] Add new version 2.1 and update GHA nightly build versions

.github/workflows/nightly-trigger.yml

@@ -31,6 +31,7 @@ jobs:
       matrix:
         branch:
           - master
+          - release-2.0
           - release-1.20
           - release-1.19
     runs-on: ubuntu-latest

flink-annotations/src/main/java/org/apache/flink/FlinkVersion.java

@@ -58,7 +58,8 @@ public enum FlinkVersion {
     v1_18("1.18"),
     v1_19("1.19"),
     v1_20("1.20"),
-    v2_0("2.0");
+    v2_0("2.0"),
+    V2_1("2.1");
 
     private final String versionStr;