Limit cart to 40 items, regardless of checkout method

[mds1]

Closes #8231

In #8145 we only limited zkSync, now we limit standard checkout also.

To test:

• Either add 45 items to your cart, OR change maxCartItems in cart.js to a number like 2. The number set there is the max allowed.
• If you change it to 2, add 2 items to your cart and things should look normal. Add a third and both checkout buttons should be disabled with a message

[thelostone-mc]

@mds1 looks like we have lint failures !

/home/travis/build/gitcoinco/web/app/assets/v2/js/cart-ethereum-zksync.js
34:30 error Unexpected trailing comma comma-dangle
173:49 error Unexpected trailing comma comma-dangle

could you get those fixed ?

[mds1]

@thelostone-mc Fixed the lint errors and rebased against master!