use net.JoinHostPort to generate host-port part of URI

[D3Hunter]

Bug Report

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1. Minimal reproduce step (Required)

see detailed description in pingcap/tiflow#6249, DM depends on dumping so need to fix it too.

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

3. What did you see instead (Required)

4. What is your TiDB version? (Required)