[WIP] add release notes v9.0.0

[hfxsd]

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[hfxsd]

Suggested change

	- 如果升级前指定过 [`format_version`](/tiflash/tiflash-configuration.md#format_version)，升级后 `format_version` 的值保持不变， TiFlash 会继续使用 `format_version` 指定的存储格式。此时如需启用新的存储格式，请在 TiFlash 配置文件中将 `format_version` 设置为 `8`。配置生效后，新写入 TiFlash 的数据将采用新的存储格式，而现有数据的存储格式则不受影响。
	- 如果升级前指定过 [`format_version`](/tiflash/tiflash-configuration.md#format_version)，升级后 `format_version` 的值保持不变，TiFlash 会继续使用 `format_version` 指定的存储格式。此时如需启用新的存储格式，请在 TiFlash 配置文件中将 `format_version` 设置为 `8`。配置生效后，新写入 TiFlash 的数据将采用新的存储格式，而现有数据的存储格式则不受影响。

[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	7db6e73	link	true	/test pull-verify

Full PR test history. Your PR dashboard.

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