cloud: update features and select-cluster tier for Premium

[qqqdan]

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[gemini-code-assist]

Code Review

This pull request introduces the new Premium plan to TiDB Cloud documentation. Key changes include adding a Premium column to the feature support table, providing a detailed description of the plan's benefits, and updating its status to public preview. Review feedback suggests verifying a potential status regression for database audit logging and recommends rewriting feature descriptions to use the second person and active voice for improved clarity.

[gemini-code-assist]

The status for "Database audit logging" on the Essential plan has changed from 🔒 (private preview) to 🚧 (under development). This seems like a feature regression. Could you please confirm if this change is intentional? According to the legend, 🚧 means "under development", which is usually a step before "private preview".

References

1. The style guide requires technical accuracy. The change from 'private preview' to 'under development' for a feature seems to be a regression and might be technically inaccurate. ^(link)

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pull-verify	50d629e	link	true	/test pull-verify

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[Oreoxmt]

Suggested change

	For mission-critical enterprise workloads requiring hyper-scale and the highest levels of performance, the Premium plan delivers a true cloud-native experience with the following features:
	- **Instant elasticity**: handles massive and unpredictable traffic spikes by scaling computing resources automatically, ensuring zero performance bottlenecks during peak demands.
	- **Unlimited growth**: architected for virtually unlimited expansion in both storage and throughput, allowing your database to scale seamlessly alongside your business without physical infrastructure constraints.
	- **Zero-infrastructure management**: provides a fully managed, "zero-touch" operational experience, eliminating the complexity of manual scaling, patching, or capacity planning.
	- **Predictable pricing**: billed based on storage and Request Capacity Units (RCUs), offering transparent, usage-based pricing that scales with your needs, so you only pay for what you use without surprises.
	- **Advanced security and compliance**: incorporates our most stringent security standards, including advanced encryption, CMEK, private networking options, and comprehensive compliance certifications to protect your most sensitive data.
	For mission-critical enterprise workloads that require large-scale capacity and consistent high performance, the Premium plan provides a cloud-native experience with the following features:
	- **Instant elasticity**: automatically scales computing resources to absorb sudden traffic spikes and maintain stable performance during peak demand.
	- **Unlimited scalability**: expands storage and throughput to keep pace with business growth, without physical infrastructure constraints.
	- **Zero-infrastructure management**: offers a fully managed service that eliminates manual scaling, patching, and capacity planning.
	- **Predictable pricing**: billed based on storage and Request Capacity Units (RCUs), offering transparent, usage-based pricing that scales with your needs, so you only pay for what you use without surprises.
	- **Advanced security and compliance**: supports advanced encryption, customer-managed encryption keys (CMEK), private networking, and compliance certifications to protect sensitive data.