refactor: standardize on shorthand for redirecting all output to /dev/null

The shorthand is more prevalent in this codebase, and is something that bash supports, so we should just use the same thing everywhere.

Note that we do not use shorthand redirect in Makefile as shell parsing is a bit different in Make and the shorthand redirect doesn't seem to be properly supported, causing CI errors.

docs/development/plugin-creation.md

@@ -103,7 +103,7 @@ if ! command -v "nginx" &>/dev/null; then
 fi
 
 # `which` example
-if ! which nginx >/dev/null 2>&1; then
+if ! which nginx &>/dev/null; then
   dokku_log_fail "Missing nginx, install it"
 fi
 ```