Add a warn log when validator API receives a request on /eth/v2/validator/blocks/*

[OisinKyne]

🎯 Problem to be solved

It is hard to debug why a node is attesting but not taking part in proposals. One reason is that they are still using the deprecated produceBlockV2 endpoint.

If we output a warn log when this method is called, (Like "Requested a block from the deprecated V2 endpoint") and a docs FAQ > Errors and Resolutions entry to explain how to fix it, it may reduce issues debugging this in future.

🛠️ Proposed solution

• Approved design doc: link
• Core team consensus on the proposed solution

Describe the solution to be implemented

🧪 Tests

• Tested by new automated unit/integration/smoke tests
• Manually tested on core team/canary/test clusters
• Manually tested on local compose simnet