Release v4.2.0-rc.0

This is Mainnet-beta Upgrade Candidate release recommended for 10% adoption.
It is also recommended for Testnet and Devnet.

https://github.com/anza-xyz/agave/blob/v4.2/CHANGELOG.md

What's Changed

• v4.2: chore(dep): backport RUSTSEC-2026-0204 crossbeam-epoch fix by @cfkelly18 in #13933
• v4.2: Use vendored rust-rocksdb fork (backport of #14008) by @mergify[bot] in #14017

Full Changelog: v4.2.0-beta.2...v4.2.0-rc.0