build(deps): bump php in /Containers/nextcloud

Bumps php from 8.3.20-fpm-alpine3.21 to 8.3.21-fpm-alpine3.21.

---
updated-dependencies:
- dependency-name: php
  dependency-version: 8.3.21-fpm-alpine3.21
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

Containers/nextcloud/Dockerfile

@@ -1,5 +1,5 @@
 # syntax=docker/dockerfile:latest
-FROM php:8.3.20-fpm-alpine3.21
+FROM php:8.3.21-fpm-alpine3.21
 
 ENV PHP_MEMORY_LIMIT=512M
 ENV PHP_UPLOAD_LIMIT=16G