Fix: Remove reference to CASHIER_MODEL enviromment variable

[localheinz]

This pull request

• removes a reference to the CASHIER_MODEL environment variable

Replaces #1562.

💁‍♂️ Running

git grep CASHIER_MODEL

on current 14x. yields

.github/CONTRIBUTING.md:9:Copy the default file using `cp phpunit.xml.dist phpunit.xml` and add the following line below the `CASHIER_MODEL` environment variable in your new `phpunit.xml` file:
UPGRADE.md:584:The `STRIPE_MODEL` environment variable has been renamed to `CASHIER_MODEL`.

so it does not exist in phpunit.xml.dist.

[driesvints]

Thanks!