This new major version of Hardhat Chai Matchers works with the new version of our hardhat-ethers plugin. If you are going to use ethers v6, this is the version that you have to install. If you are still using ethers v5, then you should keep using version 1.x of this plugin.
As part of that, we dropped support for ethers v5's BigNumbers in this version. No other breaking changes were introduced to the API.