Support connection_pool 3.x

[NotFounds]

Hello, thank you for providing this great library.

To support version 3.x, this PR relaxes the dependency for connection_pool to >= 2, < 4.

SimpleFeed only uses ConnectionPool.new(size:) and pool.with, neither of which has changed in version 3.0.

BREAKING CHANGES ConnectionPool and ConnectionPool::TimedStack now use keyword arguments rather than positional arguments everywhere. Expected impact is minimal as most people use the with API, which is unchanged.

ref. https://github.com/mperham/connection_pool/blob/main/Changes.md#300