Rename Eth1/Eth2 in documents (#3021)

## Issue Addressed

Resolves https://github.com/sigp/lighthouse/issues/3019

## Proposed Changes

- Eth2 Eth2.0 Ethereum 2.0 -> Ethereum consensus
- Eth2 network -> consensus layer
- Ethereum 2.0 specification -> Ethereum proof-of-stake consensus specification
- Eth2 deposit contract -> Staking deposit contract
- Eth1 -> execution client

## Additional Info

The description needs to be updated by someone who has permission to do. 📝 

<img width="487" alt="image" src="https://user-images.githubusercontent.com/1885716/153995211-816d9561-751e-4810-abb9-83d979379783.png">

book/src/api-vc-endpoints.md

@@ -6,7 +6,7 @@ HTTP Path | Description |
 | --- | -- |
 [`GET /lighthouse/version`](#get-lighthouseversion) | Get the Lighthouse software version.
 [`GET /lighthouse/health`](#get-lighthousehealth) | Get information about the host machine.
-[`GET /lighthouse/spec`](#get-lighthousespec) | Get the Eth2 specification used by the validator.
+[`GET /lighthouse/spec`](#get-lighthousespec) | Get the Ethereum proof-of-stake consensus specification used by the validator.
 [`GET /lighthouse/auth`](#get-lighthouseauth) | Get the location of the authorization token.
 [`GET /lighthouse/validators`](#get-lighthousevalidators) | List all validators.
 [`GET /lighthouse/validators/:voting_pubkey`](#get-lighthousevalidatorsvoting_pubkey) | Get a specific validator.
@@ -79,7 +79,7 @@ Typical Responses | 200
 
 ## `GET /lighthouse/spec`
 
-Returns the Eth2 specification loaded for this validator.
+Returns the Ethereum proof-of-stake consensus specification loaded for this validator.
 
 ### HTTP Specification