TiProxy keeps reporting errors for HAProxy health check

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. Start HAProxy and set the health check port to 6000 of TiProxy
2. Start TiProxy

2. What did you expect to see? (Required)

No errors in TiProxy

3. What did you see instead (Required)

TiProxy keeps reporting:

[2024/02/28 15:43:14.752 +08:00] [WARN] [main.proxy] [proxy/proxy.go:206] [failed to set tcp keep alive option] [connID=15] [client_addr=127.0.0.1:62842] [addr=0.0.0.0:6000] [error="failed to set keepalive and timeout: set tcp [::7f00:1]:6000->127.0.0.1:62842: setsockopt: invalid argument"]
[2024/02/28 15:43:15.157 +08:00] [WARN] [main.proxy] [proxy/proxy.go:206] [failed to set tcp keep alive option] [connID=16] [client_addr=127.0.0.1:62845] [addr=0.0.0.0:6000] [error="failed to set keepalive and timeout: set tcp 127.0.0.1:6000->127.0.0.1:62845: setsockopt: invalid argument"]
[2024/02/28 15:43:20.757 +08:00] [WARN] [main.proxy] [proxy/proxy.go:206] [failed to set tcp keep alive option] [connID=17] [client_addr=127.0.0.1:62855] [addr=0.0.0.0:6000] [error="failed to set keepalive and timeout: set tcp 127.0.0.1:6000->127.0.0.1:62855: setsockopt: invalid argument"]
[2024/02/28 15:43:23.164 +08:00] [WARN] [main.proxy] [proxy/proxy.go:206] [failed to set tcp keep alive option] [connID=18] [client_addr=127.0.0.1:62862] [addr=0.0.0.0:6000] [error="failed to set keepalive and timeout: set tcp [::7f00:1]:6000->127.0.0.1:62862: setsockopt: invalid argument"]

4. What is your version? (Required)

master

Reproduce the issue by starting HAProxy with TiProxy's health-check port set to 6000, then inspect proxy/proxy.go around line 206 and the reported TCP keepalive warning. Determine why the health check produces an invalid-argument error; done means the health check no longer causes these warnings without breaking the connection handling.