GH Actions/securitycheck: only run against PHP 5.4

As of Composer 2.9, using this type of security check is no longer needed as Composer will now natively block installation of known insecure versions of dependencies.

However, Composer 2.9 requires PHP 7.2, so we should continue to run the security check for PHP 5.4 for the time being, but we can stop running the check against PHP "latest".

Refs:
* https://blog.packagist.com/composer-2-9/
* https://github.com/composer/composer/releases/tag/2.9.0

Author: jrfnl

.github/workflows/securitycheck.yml

@@ -23,7 +23,7 @@ jobs:
 
     strategy:
       matrix:
-        php: ['5.4', 'latest']
+        php: ['5.4']
 
     steps:
       - name: Checkout code