Removed the unsupported MySQL version 5.5

[Frank945946]

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Removed the unsupported MySQL version 5.5

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.

• [x] master (the latest development version)
• [x] v7.6 (TiDB 7.6 versions)
• [x] v7.5 (TiDB 7.5 versions)
• [x] v7.4 (TiDB 7.4 versions)
• [x] v7.3 (TiDB 7.3 versions)
• [x] v7.1 (TiDB 7.1 versions)
• [x] v6.5 (TiDB 6.5 versions)
• [x] v6.1 (TiDB 6.1 versions)
• [x] v5.4 (TiDB 5.4 versions)
• [x] v5.3 (TiDB 5.3 versions)
• [x] v5.2 (TiDB 5.2 versions)
• [x] v5.1 (TiDB 5.1 versions)
• [x] v5.0 (TiDB 5.0 versions)

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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[bot]]

[LGTM Timeline notifier]

Timeline:

• 2024-04-12 06:57:16.02214087 +0000 UTC m=+1204697.549681416: :ballot_box_with_check: agreed by dveeden.

[ti-chi-bot[bot]]

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[qiancai]

@Frank945946 It looks that "MySQL 5.5" has been removed from the latest TiDB Data Migration Overview | PingCAP Docs doc. Hence, we can close this PR.