Update testnet-holesky to slashing release.

[5kyguy]

User story
testnet-holesky branch of eigenlayer-contracts repo was updated to slashing release yesterday.
Please update the testnet-holesky branch here to slashing release too.
New devs find it confusing when default branch README says testnet-holesky is for for Holesky testnet, but that branch's last commit is 10 months ago 😅

Don't Forget To

• Update the submodule pointing to testnet-holesky branch of eigenlayer-contracts.