Support migration data from source with different time_zone by default (#1670)

[ti-chi-bot]

This is an automated cherry-pick of #1670

What problem does this PR solve?

Automatically adapt source and target DB time_zone settings so users don't need to explicitly set timezone in dm config.

close #1691

What is changed and how it works?

• Deprecated the timezone setting in dm config. (maybe we should directly remove this config field)
• Always set timezone='+00:00' for all db connections in dumpling/loader/syncer.

Check List

Tests

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

Code changes

Side effects

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation
• Need to be included in the release note

[lance6716]

/lgtm

[ti-chi-bot]

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• lance6716

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[lance6716]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: a3b0244