add release notes 7.5.6

[hfxsd]

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• 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)
• v8.1 (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)

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[ti-chi-bot]

@flowbehappy: adding LGTM is restricted to approvers and reviewers in OWNERS files.

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@zanmato1984: adding LGTM is restricted to approvers and reviewers in OWNERS files.

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[stevenayu]

/lgtm

[ti-chi-bot]

@stevenayu: adding LGTM is restricted to approvers and reviewers in OWNERS files.

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/lgtm

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[hfxsd]

Preview: https://pingcap-docsite-preview-c3ndwoki8-hfxsds-projects.vercel.app/zh/tidb/dev/release-7.5.6/

[ti-chi-bot]

@zanmato1984: adding LGTM is restricted to approvers and reviewers in OWNERS files.

In response to this:

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[ti-chi-bot]

@fzzf678: adding LGTM is restricted to approvers and reviewers in OWNERS files.

In response to this:

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