Allow use of connection_pool 3.0

[sriedel]

With the introduction of non-vendored net-http-persistent, vault.gemspec also got a restriction on connection_pool that prohibits the use of connection_pool 3.

I'm not sure if this restriction is intentional or not, as net-http-persistent upstream allows use of connection_pool 3.

Since sidekiq 8.1+ requires connection_pool 3, this results in a choice to either use vault 0.19.0 and sidekiq 8.1, but be limited to ruby 3.4, or use vault 0.20.0 and ruby 4.0, but be limited to sidekiq 8.0.

It would be nice to be able to use both ruby 4.0 and sidekiq 8.1 when vault is a dependency :)

[TheDDMbassy]

Sorry if this is obvious to others, but I haven't been able to find where vault directly depends on connection_pool after 557f8c9. I'm probably missing something, but I bring it up because it might mean that resolving this issue is as simple as removing the connection_pool dependency in the gemspec altogether.

Edit: Sorry, just now seeing that #394 exists, carry on!

[sriedel]

A fix for this issue has been merged a while ago. Is there a chance to release a new version that includes this fix? :)