v8.5: bump up the latest version to v8.5.6

[hfxsd]

…9546)

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[gemini-code-assist]

Code Review

This pull request updates the documentation and version variables for the TiDB v8.5.6 release. Feedback was provided to use a relative link for the new compatibility changes entry in the upgrade guide to ensure consistency with existing documentation links.

[gemini-code-assist]

For consistency with the previous entries in this list (such as v8.5.3 and v8.5.4), please use a relative link to the release notes file instead of an absolute URL. This ensures the link remains consistent with the repository's internal linking structure and works correctly across different environments.

Suggested change

	- TiDB v8.5.6 [compatibility changes](https://docs.pingcap.com/tidb/stable/release-8.5.6/#compatibility-changes)
	- TiDB v8.5.6 [compatibility changes](/releases/release-8.5.6.md#compatibility-changes)

References

1. The style guide emphasizes consistency and readability. Using relative links for internal documentation is a best practice for maintaining a consistent linking structure within the repository. ^(link)

[ti-chi-bot]

@hfxsd: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-verify	405f9aa	link	true	/test pull-verify

Full PR test history. Your PR dashboard.

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