Use clearer `rlang::check_exclusive` for param checking in endpoints

[nmdefries]

Replace this kind of check for mutual exclusivity (appears in most of the endpoints, e.g.)

if (sum(!is.null(issues), !is.null(as_of)) > 1) {

with rlang::check_exclusive for improved readability. There's this snag though: r-lib/rlang#1647