tiproxy: fix config require-backend-tls

[djshow832]

What is changed, added or deleted? (Required)

require-backend-tls was under proxy and was true by default, but since v0.2.0 it's under security and is false by default.

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-04-11 07:39:35.253305852 +0000 UTC m=+1120836.780846398: ☑️ agreed by Oreoxmt.

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[Oreoxmt]

/approve

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.6: #17082.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.0: #17083.