Fix partially matched legend.position

[teunbrand]

Hello there,

We have been preparing a new release of ggplot2 and during a reverse dependency check, it became apparent that the prospective ggplot2 3.5.0 would break dynamAedes.

This PR updates a few partially matched legend.position arguments to the theme() function, which should fix check failures due to ggplot2's update.

To test the code changes with the release candidate, you can install it with the code below:

remotes::install_github("tidyverse/ggplot2", ref = remotes::github_pull("5592"))

The release of ggplot2 3.5.0 is scheduled for the 12th of Februari. The progress of the release can be tracked in tidyverse/ggplot2#5588. We hope that this PR might help dynamAedes get out a fix if necessary.