Update vignette preamble to use VignetteEncoding

This uses the VignetteEncoding rather than `\usepackage[utf8]{inputenc}`.  The check logic was changed in commit https://github.com/wch/r-source/commit/b1648464ee4c5fe3aa227a6151a9f23a9e868b40.  The previous template also failed R CMD check with non-ASCII vignettes because of the prefixed `%` in the encoding line, the vignette lines should have been

```r
vignette: >
  %\VignetteIndexEntry{Vignette Title}
  %\VignetteEngine{knitr::rmarkdown}
  \usepackage[utf8]{inputenc}
```