[CPS GA] billing wording

[shainaraskas]

Summary

[HOLD FOR SEP 1?]

• Updates CPS billing copy for GA: switch to present-tense, direct to the pricing table and Serverless estimator
• Adds billing start date
• Fixes https://github.com/elastic/docs-content-internal/issues/1262

Lifecycle/applies_to flip stays in https://github.com/elastic/docs-content-internal/issues/1592

Generative AI disclosure

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Tool(s) and model(s) used: grok 4.5 :(

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[eedugon]

LGTM!

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