cmd: deprecate cmd args log_level and log_encoder

[djshow832]

What problem does this PR solve?

Issue Number: ref #449

Problem Summary:
log_level and log_encoder conflict with the configs which support hot reload. This may confuse users when they update the config file.

What is changed and how it works:
Deprecate log_level and log_encoder. They will be removed in the future.

Check List

Tests

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

$ bin/tiproxy --help
start the proxy server

Usage:
  bin/tiproxy [flags]

Flags:
      --advertise-addr string   advertise address
      --config string           proxy config file path
  -h, --help                    help for bin/tiproxy
      --log_encoder string      deprecated and will be removed
      --log_level string        deprecated and will be removed
  -v, --version                 version for bin/tiproxy


$ bin/tiproxy --log_level=debug
// started normally

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.

- Deprecate the command line arguments `log_level` and `log_encoder`

[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:

• 2024-04-12 01:57:22.155316609 +0000 UTC m=+1186703.682857154: ☑️ agreed by xhebox.