Update WordPress version to 6.9-RC3 for testing

Test against WordPress 6.9 Release Candidate 3 to identify any
compatibility issues before the stable release.

Changes:
- Update Buildkite pipeline matrix to 6.9-RC3
- Update docker-compose.yml default to 6.9-RC3
- Update wordpress.Dockerfile default to 6.9-RC3

Author: oguzkocer

.buildkite/pipeline.yml

@@ -17,7 +17,7 @@ common_params:
     # - '6.4'
     # - '6.5'
     # - '6.6'
-    - '6.8.1'
+    - '6.9-RC3'
 
 steps:
   #

docker-compose.yml

@@ -4,7 +4,7 @@ services:
             context: .
             dockerfile: wordpress.Dockerfile
             args:
-                WORDPRESS_VERSION: ${WORDPRESS_VERSION:-6.8.1}
+                WORDPRESS_VERSION: ${WORDPRESS_VERSION:-6.9-RC3}
         container_name: 'wordpress'
         ports:
             - '80:80'

wordpress.Dockerfile

@@ -1,4 +1,4 @@
-ARG WORDPRESS_VERSION="6.8.1"
+ARG WORDPRESS_VERSION="6.9-RC3"
 
 FROM public.ecr.aws/docker/library/wordpress:${WORDPRESS_VERSION}