Fix Dumpling capitalization: product name should be capitalized (#21713)

[ti-chi-bot]

This is an automated cherry-pick of #21713

What is changed, added or deleted? (Required)

Changed lowercase 'dumpling' to 'Dumpling' when it refers to the product name (TiDB data export tool) rather than the command line. This prevents mistranslation of the product name as a common noun (e.g., '餃子' or '団子' in Japanese) during localization.

Command line references (e.g., tiup dumpling) remain unchanged.

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

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For details, see tips for choosing the affected versions.

• 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)
• 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: Fix Dumpling capitalization: product name should be capitalized docs#23042
• 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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