C++20 & gcc 11 compatibility

[yperbasis]

Make Silkworm compatible with gcc 11 & C++20 (C++17 is still the default).

[codecov]

Codecov Report

Merging #502 (fafe981) into master (b454586) will decrease coverage by 0.00%.
The diff coverage is 78.72%.

@@            Coverage Diff             @@
##           master     #502      +/-   ##
==========================================
- Coverage   83.30%   83.29%   -0.01%     
==========================================
  Files         149      149              
  Lines       11466    11467       +1     
==========================================
  Hits         9552     9552              
- Misses       1914     1915       +1     

Impacted Files	Coverage Δ
core/silkworm/common/util.hpp	100.00% <ø> (ø)
...de/silkworm/downloader/internals/working_chain.cpp	64.62% <72.97%> (+0.55%)	⬆️
core/silkworm/common/util.cpp	99.51% <100.00%> (ø)
node/silkworm/db/genesis.cpp	86.91% <100.00%> (ø)
...ode/silkworm/stagedsync/recovery/recovery_farm.cpp	64.03% <100.00%> (-0.08%)	⬇️
core/silkworm/consensus/base/engine.cpp	95.38% <0.00%> (-1.03%)	⬇️
core/silkworm/state/in_memory_state.cpp	91.51% <0.00%> (-0.90%)	⬇️
core/silkworm/trie/hash_builder.cpp	99.29% <0.00%> (-0.71%)	⬇️
core/silkworm/crypto/ecdsa.cpp	97.59% <0.00%> (+2.40%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b454586...fafe981. Read the comment docs.

[canepat]

LGTM

Maybe it could be worth it naming the CMake files more explicitly, i.e. rename toolchain.cmake as cpp17.cmake and move common settings in a generic toolchain.cmake included by both cpp17.cmake and cpp20.cmake. So that also CI configurations can be made more precise and intuitive