Update MySQL version references

[hfxsd]

First-time contributors' checklist

• I've signed the Contributor License Agreement, which is required for the repository owners to accept my contribution.

What is changed, added or deleted? (Required)

• MySQL 5.7 has been EOL since October 2023 and MySQL 8.0 is now reaching EOL as well.
• MySQL 8.4 and MySQL 9.7 are now the LTS versions that are under maintenance.
• Because of this it makes sense to focus more MySQL 8.0 and up.
• So replacing MySQL 8.0 with MySQL 8.x so it includes both 8.0 and 8.4.
• Also it is good to use MySQL 8.0 instead of MySQL v8.0 more consistently.

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.

• master (the latest development version)
• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 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)

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

• This PR is translated from: Update MySQL version references docs#22839
• 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

[github-actions]

Auto-sync completed successfully

Source PR: pingcap/docs#22839
Target PR: #21611

Chinese documentation has been updated based on English documentation changes.

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