db(dm): use net.JoinHostPort to generate host-port part of URI (#6248)

[ti-chi-bot]

This is an automated cherry-pick of #6248

What problem does this PR solve?

Issue Number: close #6249

What is changed and how it works?

unifies how we generate host-port part of db uri to net.JoinHostPort. we need to update dumpling(tidb repo) and go-mysql to make dm work correctly for ipv6.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
  • since CI doesn't support ipv6, test locally to use ipv6 addr for upstream & downstream
• No code

Questions

Will it cause performance regression or break compatibility?

Do you need to update user documentation, design documentation or monitoring documentation?

Release note

use `net.JoinHostPort` to generate host-port part of URI to support ipv6 address.

[ti-chi-bot]

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[D3Hunter]

/cc @lance6716 @buchuitoudegou

plz review again, since this cp changes less code

ipv6 addr tested manually

[D3Hunter]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: ba43572

[D3Hunter]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 08703ba

[D3Hunter]

/run-leak-test
/run-verify