Fix OpenGWAS API URL in DESCRIPTION

[remlapmot]

I noticed the old API URL is in the DESCRIPTION file's Description field, and as a result appears prominently on the CRAN page https://cran.r-project.org/package=ieugwasr

So this fixes that and

• bumps roxygen2 version
• updates test-coverage GHA workflow
• adds a dark mode toggle to the pkgdown site with the new lightswitch option in pkgdown
• amendment to a test
• sets: ensure files end with a newline in RStudio project file
• bumps package version

[remlapmot]

And I've hopefully now made most of the tests alot more robust to no response from the server.

[explodecomputer]

Thanks Tom!