Update warning of using private keys in .env file

[MexicanAce]

Problem
Currently, the information section for use of Private Keys in an .env file for local development states:

The private key of your wallet will be included in the .env file of the project and won't be pushed to GitHub.

We teach to always avoid private keys in .env files as it's one of the most common ways new web3 developers (experienced or not) lose funds. We even have students pledge to never put private keys with funds in them, and it's an anti-pattern that has been propagated across web3 hurting new people. pledge one and pledge 2

Proposed Update

The private key of your wallet will be included in the .env file of the project and won't be pushed to GitHub. However, as this means the private key is in plain text, it's recommended to use a private key that isn't associated with any real funds.

[shyam-patel-kira]

Since it is merged this can be closed @MexicanAce

[MexicanAce]

This issue was not resolved/merged in the associated PR, it was only mentioned. There are still areas that need to be updated in our docs with this warning

[itsacoyote]

Refer to matter-labs/zksync-web-era-docs#1031 for making the changes in zksync-docs