Set max project leaders number to 5

[arkid15r]

Very good to have this drive for consistency, and a 'nice to have' is that the validate meta file resticts the number of leaders to a maximum of 5?
Not sure if that is practical, but it is the existing limit for number of leaders (or was in 2023)

Originally posted by @jgadsden in OWASP/www-project-vulnerable-web-applications-directory#152 (comment)

[kingthorin]

Per: https://policy.owasp.org/operational/projects

Each project must have a minimum of 2 and a maximum of 5 foundation-recognized, official leaders. In the event of a resignation, leadership transition, or new leadership being appointed, a project is allowed a grace period of up to 3 months from the event to comply.