upgrade to libp2p 0.52 (#4431)

## Issue Addressed

Upgrade libp2p to v0.52

## Proposed Changes
- **Workflows**: remove installation of `protoc`
- **Book**: remove installation of `protoc`
- **`Dockerfile`s and `cross`**: remove custom base `Dockerfile` for cross since it's no longer needed. Remove `protoc` from remaining `Dockerfiles`s
- **Upgrade `discv5` to `v0.3.1`:** we have some cool stuff in there: no longer needs `protoc` and faster ip updates on cold start
- **Upgrade `prometheus` to `0.21.0`**, now it no longer needs encoding checks
- **things that look like refactors:** bunch of api types were renamed and need to be accessed in a different (clearer) way
- **Lighthouse network**
	- connection limits is now a behaviour
	- banned peers no longer exist on the swarm level, but at the behaviour level
	- `connection_event_buffer_size` now is handled per connection with a buffer size of 4
	- `mplex` is deprecated and was removed
	- rpc handler now logs the peer to which it belongs

## Additional Info

Tried to keep as much behaviour unchanged as possible. However, there is a great deal of improvements we can do _after_ this upgrade:
- Smart connection limits: Connection limits have been checked only based on numbers, we can now use information about the incoming peer to decide if we want it
- More powerful peer management: Dial attempts from other behaviours can be rejected early
- Incoming connections can be rejected early
- Banning can be returned exclusively to the peer management: We should not get connections to banned peers anymore making use of this
- TCP Nat updates: We might be able to take advantage of confirmed external addresses to check out tcp ports/ips


Co-authored-by: Age Manning <Age@AgeManning.com>
Co-authored-by: Akihito Nakano <sora.akatsuki@gmail.com>

beacon_node/lighthouse_network/src/discovery/enr.rs

@@ -7,7 +7,7 @@ use super::ENR_FILENAME;
 use crate::types::{Enr, EnrAttestationBitfield, EnrSyncCommitteeBitfield};
 use crate::NetworkConfig;
 use discv5::enr::EnrKey;
-use libp2p::core::identity::Keypair;
+use libp2p::identity::Keypair;
 use slog::{debug, warn};
 use ssz::{Decode, Encode};
 use ssz_types::BitVector;
@@ -133,7 +133,7 @@ pub fn build_or_load_enr<T: EthSpec>(
     // Build the local ENR.
     // Note: Discovery should update the ENR record's IP to the external IP as seen by the
     // majority of our peers, if the CLI doesn't expressly forbid it.
-    let enr_key = CombinedKey::from_libp2p(&local_key)?;
+    let enr_key = CombinedKey::from_libp2p(local_key)?;
     let mut local_enr = build_enr::<T>(&enr_key, config, enr_fork_id)?;
 
     use_or_load_enr(&enr_key, &mut local_enr, config, log)?;