Temporarily removed Kotlin 2.1.x branch

Because it only appeared to be working due to inadequacies.
This will be corrected separately.

Author: k163377

.github/workflows/test-main.yml

@@ -42,7 +42,7 @@ jobs:
         # LTS versions, latest version (if exists)
         java-version: [ '8', '11', '17', '21', '23' ]
         # Minimum version, latest release version, latest pre-release version (if exists)
-        kotlin: ['1.9.25', '2.0.21', '2.1.0-RC2']
+        kotlin: ['1.9.25', '2.0.21']
     env:
       KOTLIN_VERSION: ${{ matrix.kotlin }}
     name: "Kotlin ${{ matrix.kotlin }} - Java ${{ matrix.java-version }}"