docs: bump pdf stable version to release-8.1

[Oreoxmt]

Update the stable PDF version of TiDB and TiDB Cloud:

• TiDB: the stable PDF version is updated from release-7.5 to release-8.1.
• TiDB Cloud: the stable PDF version will remain at release-7.5 to ensure consistency with the current TiDB Cloud documentation.

[ti-chi-bot[bot]]

Hi @Oreoxmt. Thanks for your PR.

I'm waiting for a PingCAP-QE member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[Oreoxmt]

/hold

[ti-chi-bot[bot]]

@Yuiham: changing LGTM is restricted to collaborators

In response to [this](https://github.com/PingCAP-QE/ci/pull/2968#pullrequestreview-2072999833): > Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes/test-infra](https://github.com/kubernetes/test-infra/issues/new?title=Prow%20issue:) repository.

[Oreoxmt]

/unhold

[Oreoxmt]

@purelind @wuhuizuo Please help review and merge this PR

[ti-chi-bot[bot]]

[LGTM Timeline notifier]

Timeline:

• 2024-05-23 08:29:07.033118053 +0000 UTC m=+2333100.790253624: :ballot_box_with_check: agreed by purelind.

[ti-chi-bot[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

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This pull-request has been approved by: purelind, Yuiham

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