fix: remove explicit connection_pool dependency to allow 3.x

[chrisarcand]

Removes the explicit connection_pool ~> 2.4 runtime dependency that was preventing use of connection_pool 3.x (required by sidekiq 8.1+).

The gem is already managed as a transitive dependency by net-http-persistent 4.0.8, which supports connection_pool >= 2.2.4, < 4. Since we don't directly use connection_pool's API, there's no need to declare it explicitly.

Fixes #393

[nevans]

FWIW, there is still the ugly timeout hack that reaches into the connection pool's internals, and is more likely to break between major versions.

        @nhp = Net::HTTP::Persistent.new(name: "vault-ruby", pool_size:)
        @nhp.pool.instance_variable_set(:@timeout, pool_timeout)

That doesn't need to block this PR... but it might be worth converting that into an upstream PR (or two).

[nevans]

Related, dependabot can be configured to "widen" the gem deps. That should be able to automatically create PRs like this:

- spec.add_runtime_dependency "connection_pool",     "~> 2.4"
+ spec.add_runtime_dependency "connection_pool",     ">= 2.4, < 4"

(Obviously, it'd be better to simply remove the direct dep on connection_pool internals!)