fix(protocol): handle ElectLeaders V1 non-flexible headers

[DCjanus]

Why

ElectLeaders request/response version 1 must use the non-flexible wire format.

That path was fixed once in #3312, but a later refactor left the ElectLeaders header version selection inconsistent with the protocol version again. As a result, a client configured for V2_3_0_0 can fail to decode ElectLeaders V1 responses from newer brokers with:

kafka: insufficient data to decode packet, more bytes expected

What

• add a functional test that covers low-version client compatibility against newer brokers for the ElectLeaders V1 non-flexible path
• select ElectLeaders request/response header versions from the actual protocol version instead of treating V1 as flexible

Testing

This PR contains two commits. The first commit only adds a functional test and is expected to fail. The second commit only contains the protocol fix and is expected to make CI pass.

Before opening this PR, I created a draft PR in my fork to run CI and verify that the test was actually effective.

• failing job: FVT (PR) / Kafka 2.6.3
• passing job: FVT (PR) / Kafka 2.6.3