Restore operator quotations and update to roxygen2 7.3.1

[nanxstats]

This PR reverts the changes in #314 by adding the quotes back as the parser bug in roxygen2 7.3.0 has been fixed in roxygen2 7.3.1 (on CRAN 2024-01-22), and reran roxygen2 to update the RoxygenNote field in DESCIRPTION to use 7.3.1.

Changing this back ensures the convention to import special operators like := or %>% is consistent with both

1. The generated NAMESPACE file in this package.
2. All other packages that have been quoting them all along.

so I think this is worth it.

[nanxstats]

Note that although running this using roxygen2 7.3.0 will result in those parsing errors, this message will be shown at the top:

ℹ Updating gsDesign2 documentation
✖ Installed roxygen2 is older than the
  version used with this package
ℹ You have "7.3.0" but you need "7.3.1"

which tells people what to do - upgrade.