feat: update genesis for holesky

[Rjected]

Updates the genesis holesky, using the genesis.json from:
https://github.com/eth-clients/holesky/blob/ea39b9006210848e13f28d92e12a30548cecd41d/custom_config_data/genesis.json

Updates the Paris fork_block docs to note that holesky uses mergeNetSplitBlock in its genesis.

Also updates the datadir resolution to use each named chain's to_string method, so additional testnets will not need special handling.

[codecov]

Codecov Report

Merging #4773 (ffe5ae1) into main (371388d) will decrease coverage by 0.01%.
Report is 1 commits behind head on main.
The diff coverage is 94.73%.

Files	Coverage Δ
crates/primitives/src/chain/spec.rs	98.08% <100.00%> (ø)
bin/reth/src/dirs.rs	75.34% <93.33%> (+4.32%)	⬆️

... and 9 files with indirect coverage changes

Flag	Coverage Δ
integration-tests	16.51% <0.00%> (-0.01%)	⬇️
unit-tests	63.39% <94.73%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
reth binary	32.23% <93.33%> (+0.15%)	⬆️
blockchain tree	83.69% <ø> (ø)
pipeline	88.53% <ø> (ø)
storage (db)	73.49% <ø> (ø)
trie	94.48% <ø> (ø)
txpool	49.51% <ø> (ø)
networking	76.85% <ø> (-0.09%)	⬇️
rpc	57.68% <ø> (+<0.01%)	⬆️
consensus	62.84% <ø> (ø)
revm	28.33% <ø> (ø)
payload builder	8.20% <ø> (ø)
primitives	86.52% <100.00%> (ø)