*: Rename table placement_policies to placement_rules

[lcwangchao]

What problem does this PR solve?

Issue Number: close #31425

Problem Summary:

The modification is from doc: https://github.com/pingcap/tidb/blob/master/docs/design/2020-06-24-placement-rules-in-sql.md#information_schemaplacement_policies

What is changed and how it works?

Check List

Tests

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

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

`information_schema.placement_rules` is renamed to `information_schema.placement_policies`

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• morgo
• xhebox

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[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/499c774cefead140b922433067a3d4cbba7a8cfb

[morgo]

/run-all-tests

[xhebox]

Also close #31425

[xhebox]

/merge

[ti-chi-bot]

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

Commit hash: 76e3e79

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@lcwangchao: Your PR was out of date, I have automatically updated it for you.

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