Changes

• Added explicit dependencies on R (>= 4.3.0), Rcpp (>= 1.1.0), and RcppArmadillo (>= 15.0.2-2)
• Removed CXX11 from src/Makevars and src/Makevars.win to avoid potential compilation issues
  with newer versions of Armadillo through RcppArmadillo.
• Switched README.Rmd to README.qmd to use Quarto for rendering.
• Fixed CITATION file to use c() instead of personList() and bibentry() to
  avoid CRAN check notes.
• Updated GitHub Action workflows.