statistics: mark NDV as deprecated and update variable scopes

[Oreoxmt]

What is changed, added or deleted? (Required)

• updated the description for the NDV field from the histogram bucket.
• corrected some variable's scopes.

Which TiDB version(s) do your changes apply to? (Required)

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By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.

For details, see tips for choosing the affected versions.

• master (the latest development version)
• v8.3 (TiDB 8.3 versions)
• v8.2 (TiDB 8.2 versions)
• v8.1 (TiDB 8.1 versions)
• v8.0 (TiDB 8.0 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)
• v5.3 (TiDB 5.3 versions)
• v5.2 (TiDB 5.2 versions)

What is the related PR or file link(s)?

• This PR is translated from: statistics: mark NDV as deprecated and update variable scopes docs#18499
• Other reference link(s):

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[Oreoxmt]

/cc @hi-rustin

[qiancai]

Rest LGTM

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-08-07 08:03:23.894874231 +0000 UTC m=+425533.761973318: ☑️ agreed by qiancai.

[Rustin170506]

Looks good to me. Thanks! 👍

[ti-chi-bot]

@hi-rustin: adding LGTM is restricted to approvers and reviewers in OWNERS files.

In response to this:

Looks good to me. Thanks! 👍

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[Oreoxmt]

/approve

[ti-chi-bot]

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.2: #18319.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #18320.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.0: #18321.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #18322.