Migrate test_rpc_subscriptions to tpu-client-next

[alexpyattaev]

Problem

Several RPC tests in rpc-test/tests/rpc.rs still rely on UDP TPU path, which is deprecated. In order to remove the TPU UDP path, we need to migrate the tests.

Proposed Solution

• Migrate the test_rpc_subscriptions test to use tpu-client-next
• Remove test_tpu_send_transaction test
• Remove UDP path from run_tpu_send_transaction

[AvhiMaz]

on it 🫡