Enforce required PHP version and Add Security Advisories

[aaronsaray]

In its current construction, this app supports php 7.3 and above I believe. Looking at the open tickets and testing, it doesn't look like it supports PHP 8 yet.

Additionally, the roave/security-advisories composer dependency provides other limitations in the form of conflicts to stop insecure library additions.

I suggest:

• add php 7.3+ to the composer php requirements
• add the roave security advisories branch