Bump NGINX Plus version to 26.

contrib/src/nginx/version

diff --git a/contrib/src/nginx/version b/contrib/src/nginx/version
index fa9e345..c084ebe 100644
--- a/contrib/src/nginx/version
+++ b/contrib/src/nginx/version
@@ -1,2 +1,2 @@
 NGINX_VERSION := 1.21.6
-NGINX_PLUS_VERSION := 25
+NGINX_PLUS_VERSION := 26