add the removal reason for tidb_enable_change_multi_schema

[qiancai]

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What is changed, added or deleted? (Required)

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• master (the latest development version)
• v8.5 (TiDB 8.5 versions)
• v8.4 (TiDB 8.4 versions)
• v8.3 (TiDB 8.3 versions)
• v8.2 (TiDB 8.2 versions)
• v8.1 (TiDB 8.1 versions)
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• 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)

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

• This PR is translated from: add the removal reason for tidb_enable_change_multi_schema docs-cn#19169
• 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

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[qiancai]

/approve

[ti-chi-bot]

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #19527.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #19528.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #19529.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.4: #19530.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #19531.