Shanghai Update (#906)

* eip-3651

* eip-3855

* eip-3860

* eip-4895

* eip-6049

* shanghai

* eip-3651-alexprut-review

* eip-3860-alexprut-review

* eip-4895-alexprut-review

* eip-6049-alexprut-review

* shanghai-alexprut-review

* update README.md & BRANCHES.md

BRANCHES.md

@@ -4,7 +4,8 @@ Each protocol version is specified in `Paper.tex` found in a branch of this repo
 
 | Branch            | Version                                                           | Applicable Block Numbers        |
 |-------------------|-------------------------------------------------------------------------------------------------------------------|-----------------------------------|
-| master               | [Paris](https://github.com/ethereum/execution-specs/blob/master/network-upgrades/mainnet-upgrades/paris.md)    | 15,537,394 until 17,034,869
+| master               | [Shanghai](https://github.com/ethereum/execution-specs/blob/master/network-upgrades/mainnet-upgrades/shanghai.md)    | 17,034,870 until 19,426,586
+| paris               | [Paris](https://github.com/ethereum/execution-specs/blob/master/network-upgrades/mainnet-upgrades/paris.md)    | 15,537,394 until 17,034,869
 | london            | [Gray Glacier](https://github.com/ethereum/execution-specs/blob/master/network-upgrades/mainnet-upgrades/gray-glacier.md) <br> [Arrow Glacier](https://github.com/ethereum/execution-specs/blob/master/network-upgrades/mainnet-upgrades/arrow-glacier.md) <br> [London](https://github.com/ethereum/eth1.0-specs/blob/master/network-upgrades/mainnet-upgrades/london.md)    |   Since 15,050,000 until 15,537,393 <br> Since 13,773,000 until 15,049,999 <br> Since 12,965,000 until 13,772,999 |
 | berlin            | [Berlin](https://github.com/ethereum/eth1.0-specs/blob/master/network-upgrades/mainnet-upgrades/berlin.md)        | Since 12,244,000 until 12,964,999 |
 | istanbul          | [Muir Glacier](https://eips.ethereum.org/EIPS/eip-2387) <br> [Istanbul](https://eips.ethereum.org/EIPS/eip-1679)  | Since 9,200,000 until 12,243,999 <br> Since 9,069,000 until 9,199,999 |