Pin PHP 7.4.23 for zenerre Jenkins CI agent

Summary:
The Jenkins PHP agent image now uses PHP 8.1.5.

Some application like Auth Grove or tools. still run under PHP 7
in production. Probably best to keep an agent with PHP 7.4 to ease
the upgrade process to PHP 8.x

Ref T1677

Test Plan: salt-call --local state.sls roles/paas-docker/containers/jenkins_agent

Reviewers: DorianWinty, dereckson

Reviewed By: dereckson

Maniphest Tasks: T1677

Differential Revision: https://devcentral.nasqueron.org/D2681