Update staking pool creation

[brunomenezes]

📄 Context

Currently, we offer two commission models when creating a staking pool. that is, Gas-based and Flat-rate commissions. Based on the usage of both models, we will streamline and simplify to only offer the Flat-rate based. We currently have only one pool using Gas-based on mainnet that has no users and is "not active".

✔️ Solution

Remove the Gas-based option in the Commission Model step in the node-runners staking pool creation feature.

📈 Subtasks

• Remove the Gas-based option.
• Validations are updated as required.
• Update tests related to that step.
• Update stories, if any.

🎯 Definition of Done

• The staking pool creation flow is functioning as expected.
• CI checks are passing.
• Vercel deployments have no problems.