Update Contributing file

[clauBv23]

In the contributing file says that should use pnpm version 7 https://github.com/dethcrypto/TypeChain/blob/33879dcb8f973e34edfa23f05340428699a30bf8/CONTRIBUTING.md?plain=1#L7-L8 meanwhile, in the package.json file, it is forced to be bigger than v8.0.0. https://github.com/dethcrypto/TypeChain/blob/33879dcb8f973e34edfa23f05340428699a30bf8/package.json#L5-L7

It also says that npm version shouldn't be newer than v6 https://github.com/dethcrypto/TypeChain/blob/33879dcb8f973e34edfa23f05340428699a30bf8/CONTRIBUTING.md?plain=1#L16-L17

due to an issue that is already solved