ci: add r2021b

Author: scivision

.github/workflows/ci.yml

@@ -26,7 +26,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-22.04, windows-2022]
-        release: [R2022a, R2022b, R2023a, R2023b, R2024a, R2024b, R2025a]
+        release: [R2021b, R2022a, R2022b, R2023a, R2023b, R2024a, R2024b, R2025a]
 # on Linux, even R2025a needs GCC 10.x and hence Github Actions runner ubuntu-22.04
         include:
         - os: macos-latest