v1.2.1 - new CRAN release

PLNmodels 1.2.1 (2025-03-10)

• fix NOTES in CRAN due to missing packages in \link{} (PR #142)
• Now requires R >= 4.1.0 because package code uses the pipe |> (PR #142)
• fix sandwich variance estimation (PR #140)
• fix use of native pipe to ensure compatibility with R 3.6 (merge PR #125, fix #124)