net, backend: fix wrong format in error packet

[djshow832]

What problem does this PR solve?

Issue Number: close #356

Problem Summary:
Wrong format: ERROR 1105 (HY000): Unknown error%!(EXTRA string=Verify TiDB capability failed, please upgrade TiDB)

What is changed and how it works:

• Use NewDefaultError and NewError in github.com/go-mysql-org/go-mysql/mysql because they handle both formatted and unformatted error messages.
• Replace some constants from tidb/parser/mysql to proxy/net or github.com/go-mysql-org/go-mysql/mysql.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Notable changes

• Has configuration change
• Has HTTP API interfaces change
• Has tiproxyctl change
• Other user behavior changes

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: xhebox

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-09-04 09:02:56.142217688 +0000 UTC m=+2349740.691233674: ☑️ agreed by xhebox.