[FLINK-35682][release] Add new version 2.0 and update GHA nightly build versions

Author: reswqa

Diff for: .github/workflows/nightly-trigger.yml

@@ -31,6 +31,7 @@ jobs:
       matrix:
         branch:
           - master
+          - release-1.20
           - release-1.19
           - release-1.18
     runs-on: ubuntu-latest

Diff for: flink-annotations/src/main/java/org/apache/flink/FlinkVersion.java

@@ -57,7 +57,8 @@ public enum FlinkVersion {
     v1_17("1.17"),
     v1_18("1.18"),
     v1_19("1.19"),
-    v1_20("1.20");
+    v1_20("1.20"),
+    v2_0("2.0");
 
     private final String versionStr;