rpc: Change max internal rpc connection count limit

[StephanDollberg]

Previously we limited this to 32 for fear of connection explosion on larger core machines but we haven't seen any issues so far and 64 core machines (e.g.: AWS 16xlarge) are becoming more common so up the default to account for this.

Having one per shard is important for even load distribution across shards.

Backports Required

• [x] none - not a bug fix
• [ ] none - this is a backport
• [ ] none - issue does not exist in previous branches
• [ ] none - papercut/not impactful enough to backport
• [ ] v24.3.x
• [ ] v24.2.x
• [ ] v24.1.x

Release Notes

• none

[vbotbuildovich]

the below tests from https://buildkite.com/redpanda/redpanda/builds/58286#019345cc-0e62-44eb-b1ec-66f4babd9363 have failed and will be retried

gtest_raft_rpunit

[StephanDollberg]

/backport v24.3

[vbotbuildovich]

Oops! Something went wrong.

Workflow run logs.

[StephanDollberg]

/backport v24.3.x

[StephanDollberg]

/backport v24.2.x