php8.2 fix undeclared properties on backoffice contribution form

php8.2 fix undeclared properties on backoffice contribution form

There are just 2 properties

_payNow is distinctly internal, and easily searched in universe so I made it private

- _contributionID is specifically set for the purposes of hooks - although I expect that
was more relevant when the code was shared with the front end form. However, I have
set it up to still work for now, albeit with a deprecation notice