system-variables: ensure alphabetical order (#5711)

[ti-chi-bot]

This is an automated cherry-pick of #5711

What is changed, added or deleted? (Required)

The system variables are not in alphabetical order. This is useful for users when a list is long because it makes it easier to search for them. I also plan to auto-generate the system-variables page from the server's variables list, but it's not possible to do when the order is non-deterministic.

I have made one other small edit, which is to normalize the introduced version and remove the text " version" when it appears. I will fix the CI if this breaks any anchors.

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

• master (the latest development version)
• v5.1 (TiDB 5.1 versions)
• v5.0 (TiDB 5.0 versions)
• v4.0 (TiDB 4.0 versions)
• v3.1 (TiDB 3.1 versions)
• v3.0 (TiDB 3.0 versions)
• v2.1 (TiDB 2.1 versions)

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• This PR is translated from:
• 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

[ti-chi-bot]

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