config: move require-backend-tls from proxy to security

[djshow832]

What problem does this PR solve?

Issue Number: close #411

Problem Summary:

• require-backend-tls is more like a security config
• require-backend-tls is true by default, which causes some compatible problems when updating it

What is changed and how it works:

• Move require-backend-tls from proxy section to security
• Change the default value of require-backend-tls to false

Check List

Tests

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

tiup playground v7.5.0 --tiproxy 1 --tiproxy.binpath /Users/zhangming/gopath/src/github.com/pingcap/tiproxy/bin/tiproxy --tiproxy.config cfg --tiflash 0

cat cfg
[proxy]
  require-backend-tls = false

This proves that it's compatible with the previous config.

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

[djshow832]

/retest

[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:

• 2023-12-22 09:36:54.869577937 +0000 UTC m=+1212905.906804863: ☑️ agreed by xhebox.