Update dependencies flagged by security advisories and require PHP 8.4

Bumps the packages reported by CVE/GHSA scans in #1564 to their patched
releases (with transitive dependencies):

- symfony/http-foundation 7.3.1 -> 7.4.14 (CVE-2025-64500 / GHSA-3rg7-wf37-54rm)
- phpunit/phpunit 10.5.47 -> 10.5.64 (CVE-2026-24765 / GHSA-vvj3-c3rp-c85p)
- aws/aws-sdk-php 3.349.3 -> 3.388.0 (GHSA-27qh-8cxx-2cr5)
- enshrined/svg-sanitize 0.21.0 -> 0.22.0 (GHSA-22wq-q86m-83fh)

Some transitive dependencies now require PHP 8.4, which matches the runtime
shipped in the official LinuxServer image, so the composer requirement is
raised to ^8.4, CI is pinned to PHP 8.4, and the readme is updated to match.

The remaining advisories in the report (php84, curl, libpq, git, sqlite,
busybox, coreutils) come from the LinuxServer base image, not this
repository, and are addressed by rebuilding the image on an updated base.

Refs #1564

.github/workflows/ci.yml

@@ -12,7 +12,7 @@ jobs:
       - name: Setup PHP, with composer and extensions
         uses: shivammathur/setup-php@v2 #https://github.com/shivammathur/setup-php
         with:
-          php-version: '8.3'
+          php-version: '8.4'
           extensions: mbstring, dom, fileinfo, mysql, libxml, xml, xmlwriter, dom, tokenizer, filter, json, phar, pcre, openssl, pdo, intl, curl
 
       - name: Cache composer dependencies