Bump NGINX Plus version to 24.

contrib/src/nginx/version

diff --git a/contrib/src/nginx/version b/contrib/src/nginx/version
index 4dfa7e0..e082376 100644
--- a/contrib/src/nginx/version
+++ b/contrib/src/nginx/version
@@ -1,2 +1,2 @@
 NGINX_VERSION := 1.19.9
-NGINX_PLUS_VERSION := 23
+NGINX_PLUS_VERSION := 24