Remove R4.1 pipes

[zhizhongpu]

In this issue we remove |>s so that the package is compatible with R versions prior to 4.1.

[zhizhongpu]

Summary

We:

• remove pipes that are supported only on R>=4.1, to comply with the existing requirement of R>=3.5

Latest state of issue branch
Release is https://github.com/JMSLab/eventstudyr/releases/tag/v1.2.0