tidb: add description about GB18030

[CbcWestwolf]

What is changed, added or deleted? (Required)

Add description about GB18030

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

Tips for choosing the affected version(s):

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 (in Chinese).

• master (the latest development version)
• v9.0 (TiDB 9.0 versions)
• 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)
• 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)

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

• This PR is translated from:
• Other reference link(s): Do NOT merge this PR until charset,collation: implement GB18030 and 2 collations tikv/tikv#17467 and charset,collation: implement GB18030 and 2 collations | tidb-test=pr/2087 tidb#55792 are merged

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

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-09-23 13:32:44.88699319 +0000 UTC m=+1486434.627417141: ☑️ agreed by hfxsd.

[shawn0915]

@CbcWestwolf 请问这里的GB18030是指 GB 18030-2022 么？

[CbcWestwolf]

请问这里的GB18030是指 GB 18030-2022 么？

是的 @shawn0915

[shawn0915]

请问这里的GB18030是指 GB 18030-2022 么？

是的 @shawn0915

这是之前企业版支持的特性？要不要明确说明一下是 GB 18030-2022 毕竟跟老国标是有区别的。

[lilin90]

@CbcWestwolf Would you please update the corresponding descriptions in this PR from v8.4 to v9.0, and add description about #18662 (comment)?

[CbcWestwolf]

@lilin90 Sure, let me update it after merging the code.

[ti-chi-bot]

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[CbcWestwolf]

/retest