Dapp detection should now use window.ethereum and not window.web3

[bbondy]

Description

On November 16, 2020 MetaMask plans to remove window.web3 MetaMask/metamask-extension#8077 in favor of window.ethereum

We should remove our Dapp detection for window.web3 and use window.ethereum instead.

QA test plan

1. Fresh profile
2. Go to cryptokitties.co
3. Ensure the dapp detection prompt still comes up

[btlechowski]

Verification passed on

Brave	1.16.55 Chromium: 86.0.4240.72 (Official Build) dev (64-bit)
Revision	581582174c512f44f44fd1aea340471f54b2365f-refs/branch-heads/4240@{#1134}
OS	Ubuntu 18.04 LTS

Verified test plan from the description
Verified the info bar is shown. Verified that the injected script uses window.ethereum

[srirambv]

Verification passed on

Brave	1.16.59 Chromium: 86.0.4240.75 (Official Build) beta (x86_64)
Revision	c69c33933bfc72a159aceb4aeca939eb0087416c-refs/branch-heads/4240@{#1149}
OS	macOS Version 10.15.7 (Build 19H2)

• Verified dapp detection script uses window.ethereum instead of window.web3
  

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Verification passed on

Brave	1.16.59 Chromium: 86.0.4240.75 (Official Build) beta (64-bit)
Revision	c69c33933bfc72a159aceb4aeca939eb0087416c-refs/branch-heads/4240@{#1149}
OS	Windows 10 OS Version 1809 (Build 17763.1490)

• Verified dapp detection script uses window.ethereum instead of window.web3