Incorrect version annotation for `max_concurrent_sftp_sessions` in KeyPairResourcePolicy

## Description

In the `src/ai/backend/manager/api/schema.graphql` file, the `max_concurrent_sftp_sessions` field within the `KeyPairResourcePolicy` (not `CreateKeyPairResourcePolicyInput`, `ModifyKeyPairResourcePolicyInput`) is incorrectly annotated as being introduced in version 23.03.3, whereas it was actually introduced in version 24.03.4. This discrepancy may lead to unexpected errors in the UI when displaying version-dependent features.

## Expected Behavior

The annotation should accurately reflect the version in which `max_concurrent_sftp_sessions` was introduced, ensuring that UI components that depend on version information function correctly without causing unexpected errors.

## Suggested Fix

Update the version annotation for `max_concurrent_sftp_sessions` in the `KeyPairResourcePolicy` (not in `CreateKeyPairResourcePolicyInput` or `ModifyKeyPairResourcePolicyInput`) from 23.03.3 to 24.03.4 in the `src/ai/backend/manager/api/schema.graphql` file.

## Additional Context

This issue could affect the way features are displayed or enabled in the UI, potentially leading to user confusion or errors if the UI relies on version information to determine feature availability. It is important to clarify that the correction is needed specifically for the `KeyPairResourcePolicy`, not for the related input types..

JIRA Issue: BA-473