planner: unify the terms NDV and cardinality in the optimizer (#26345)

[ti-srebot]

cherry-pick #26345 to release-5.0
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---

What problem does this PR solve?

Issue Number: close #26283

Problem Summary: planner: unify the terms NDV and cardinality in the optimizer

What is changed and how it works?

In the optimizer, both terms NDV and cardinality are used to refer to the same meaning the number of distinct values.
In many other DBs, cardinality also means the estimated number of rows returned by a step or an operator.
So to make it clear and unified, we decided to remove cardinality and only use NDV to refer to this meaning in the optimizer.

No logical change, just rename some functions and variables.

Release note

• No release note

[ti-srebot]

/run-all-tests

[ti-srebot]

@qw4990 you're already a collaborator in bot's repo.

[qw4990]

/run-all-tests

[ti-chi-bot]

[REVIEW NOTIFICATION]

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[qw4990]

/run-unit-test

[qw4990]

PTAL @zhouqiang-cl

[zhouqiang-cl]

/merge

[ti-chi-bot]

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

Commit hash: 2c06431

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