Enable proposer boost on mainnet and GBC (sigp#3131)

## Proposed Changes

Mitigate the fork choice attacks described in [_Three Attacks on Proof-of-Stake Ethereum_](https://arxiv.org/abs/2110.10086) by enabling proposer boost @ 70% on mainnet.

Proposer boost has been running with stability on Prater for a few months now, and is safe to roll out gradually on mainnet. I'll argue that the financial impact of rolling out gradually is also minimal.

Consider how a proposer-boosted validator handles two types of re-orgs:

## Ex ante re-org (from the paper)

In the mitigated attack, a malicious proposer releases their block at slot `n + 1` late so that it re-orgs the block at the slot _after_  them (at slot `n + 2`). Non-boosting validators will follow this re-org and vote for block `n + 1` in slot `n + 2`. Boosted validators will vote for `n + 2`. If the boosting validators are outnumbered, there'll be a re-org to the malicious block from `n + 1` and validators applying the boost will have their slot `n + 2` attestations miss head (and target on an epoch boundary). Note that all the attesters from slot `n + 1` are doomed to lose their head vote rewards, but this is the same regardless of boosting.

Therefore, Lighthouse nodes stand to miss slightly more head votes than other nodes if they are in the minority while applying the proposer boost. Once the proposer boost nodes gain a majority, this trend reverses.

## Ex post re-org (using the boost)

The other type of re-org is an ex post re-org using the strategy described here: sigp#2860. With this strategy, boosted nodes will follow the attempted re-org and again lose a head vote if the re-org is unsuccessful. Once boosting is widely adopted, the re-orgs will succeed and the non-boosting validators will lose out.

I don't think there are (m)any validators applying this strategy, because it is irrational to attempt it before boosting is widely adopted. Therefore I think we can safely ignore this possibility.

## Risk Assessment

From observing re-orgs on mainnet I don't think ex ante re-orgs are very common. I've observed around 1 per day for the last month on my node (see: https://gist.github.com/michaelsproul/3b2142fa8fe0ff767c16553f96959e8c), compared to 2.5 ex post re-orgs per day.

Given one extra slot per day where attesting will cause a missed head vote, each individual validator has a 1/32 chance of being assigned to that slot. So we have an increase of 1/32 missed head votes per validator per day in expectation. Given that we currently see ~7 head vote misses per validator per day due to late/missing blocks (and re-orgs), this represents only a (1/32)/7 = 0.45% increase in missed head votes in expectation. I believe this is so small that we shouldn't worry about it. Particularly as getting proposer boost deployed is good for network health and may enable us to drive down the number of late blocks over time (which will decrease head vote misses).

## TL;DR

Enable proposer boost now and release ASAP, as financial downside is a 0.45% increase in missed head votes until widespread adoption.

common/eth2_network_config/built_in_network_configs/gnosis/config.yaml

@@ -74,9 +74,8 @@ CHURN_LIMIT_QUOTIENT: 4096
 
 # Fork choice
 # ---------------------------------------------------------------
-# TODO: enable once proposer boosting is desired on mainnet
 # 70%
-# PROPOSER_SCORE_BOOST: 70
+PROPOSER_SCORE_BOOST: 70
 
 # Deposit contract
 # ---------------------------------------------------------------

common/eth2_network_config/built_in_network_configs/mainnet/config.yaml

@@ -74,9 +74,8 @@ CHURN_LIMIT_QUOTIENT: 65536
 
 # Fork choice
 # ---------------------------------------------------------------
-# TODO: enable once proposer boosting is desired on mainnet
 # 70%
-# PROPOSER_SCORE_BOOST: 70
+PROPOSER_SCORE_BOOST: 70
 
 # Deposit contract
 # ---------------------------------------------------------------

consensus/types/src/chain_spec.rs

@@ -500,7 +500,7 @@ impl ChainSpec {
              * Fork choice
              */
             safe_slots_to_update_justified: 8,
-            proposer_score_boost: None,
+            proposer_score_boost: Some(70),
 
             /*
              * Eth1
@@ -698,7 +698,7 @@ impl ChainSpec {
              * Fork choice
              */
             safe_slots_to_update_justified: 8,
-            proposer_score_boost: None,
+            proposer_score_boost: Some(70),
 
             /*
              * Eth1