releases: add tidb 5.1.2 release notes (#6500)#6554

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qiancai merged 1 commit intopingcap:release-5.1from ti-chi-bot:cherry-pick-6500-to-release-5.1

Sep 27, 2021

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Commits on Sep 27, 2021

releases: add tidb 5.1.2 release notes (pingcap#6500 )

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