security: report warning when TLS version is below 1.2

[djshow832]

What problem does this PR solve?

Issue Number: close #337

Problem Summary:
TiDB reports warnings when TLS version is below 1.2: https://github.com/pingcap/tidb/blob/master/util/misc.go#L499
But TiProxy doesn't: https://github.com/pingcap/TiProxy/blob/main/lib/config/proxy.go#L114

TiProxy can add the warning to keep consistency.

What is changed and how it works:

• Move parsing TLS version from config to security because it can import logger.
• Report warning if necessary.

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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• OWNERS [xhebox]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-08-30 08:59:54.947101293 +0000 UTC m=+1917559.496117277: ☑️ agreed by xhebox.