feat: deprecate new deployments

[oddgrd]

Description of change

As of the 1st of January 2025, making new deployments on the shuttle.rs platform has been deprecated, and users will be directed to upgrade to the shuttle.dev platform.

How has this been tested? (if applicable)

Will test in staging.

[greptile-apps]

PR Summary

This PR updates the deprecation messaging for the shuttle.rs platform, focusing on two key changes:

• Modified gateway/src/service.rs to update project creation deprecation message from "is deprecated" to "has been deprecated" as of December 19th, 2024
• Added deprecation handling in gateway/src/api/latest.rs to block new deployments with a notice effective January 1st, 2024
• Maintains existing functionality while directing users to migrate to shuttle.dev platform

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[greptile-apps]

PR Summary

(updates since last review)

This PR continues the deprecation of shuttle.rs by removing PostHog analytics integration and updating related infrastructure.

• Removed async-posthog dependency and all PostHog client code from gateway/Cargo.toml and related files
• Simplified override_create_service in gateway/src/api/latest.rs by removing unused parameters and state
• Removed PostHog client initialization and usage from gateway/src/main.rs start function

The changes focus on cleaning up analytics tracking while maintaining the core deprecation functionality previously implemented.

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