releases: fix transaction_atomicity spelling to match the TOML config key

[yahonda]

What is changed, added or deleted? (Required)

release-6.6.0.md's changelog entry for issue #7935 says `transaction_atomicity` (underscore), but this specific fix was about the field failing to update via the configuration file — and the TOML config file field is `transaction-atomicity` (hyphen). The same historical changelog entry appears in release-6.1.4.md and release-6.5.1.md, both already correctly spelled with a hyphen; only release-6.6.0.md has the underscore typo.

Verification:

• Original bug report (tiflow#7935): titled "can't update transaction-atomicity via configuration file", with a repro using `[sink]\ntransaction-atomicity = "none"` in a TOML config file.
• Fixing PRs (tiflow#7980, Update the link of privacy policy #8022, Update the link of privacy policy #8023): all titled "support update transaction-atomicity and protocol via config file".
• Source: `pkg/config/sink.go` defines `TxnAtomicity *AtomicityLevel `toml:"transaction-atomicity" json:"transaction-atomicity,omitempty"`` — the TOML tag is hyphenated. (`transaction_atomicity`, underscore, is a separate field in `cdc/api/v2/model.go`, used only for the OpenAPI v2 JSON API.)
• Live test: deployed a TiCDC v8.5.8 cluster and created a changefeed with `--config` pointing to a TOML file containing `[sink]\ntransaction-atomicity = "table"` — accepted. The same file with `transaction_atomicity = "none"` (underscore) was rejected with: `Error: ... contained unknown configuration options: sink.transaction_atomicity`.

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

• 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)

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

• This PR is translated from:
• Other reference link(s): tiflow#7980, pkg/config/sink.go

AI agent involvement

• The changes in this PR were primarily made by an AI agent on behalf of the PR author.

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

Summary by CodeRabbit

• Documentation
  • Updated TiCDC release note terminology to use the correct hyphenated configuration parameter names: transaction-atomicity and protocol.

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📝 Walkthrough

Walkthrough

The TiCDC bug-fix entry in the 6.6.0 release notes now uses transaction-atomicity and protocol as configuration parameter names.

Changes

Release note parameter naming

Layer / File(s)	Summary
Update TiCDC parameter names releases/release-6.6.0.md	The bug-fix entry uses the hyphenated transaction-atomicity parameter name instead of transaction_atomicity.

Priority: ⬇️ Low

Estimated code review effort: 1 (Trivial) | ~2 minutes

Change: Other

Suggested reviewers: qiancai

Merge Risk: ⚪ Minimal · up to 64846

This documentation-only change has no runtime impact and is ready to merge.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly identifies the release-note change and the correction from the underscore form to the TOML configuration key spelling.
Description check	✅ Passed	The description covers the required change, affected version, reference links, AI involvement, and applicable change categories. It provides sufficient technical context and verification details.

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