perf: Optimize i18n instance caching in App Router

[hbjORbj]

What does this PR do?

1. Added module-level caching for i18n instances to prevent unnecessary recreation
2. Confirmed that there's no instance of getFixedT util

• This change improves translation loading performance by reusing i18n instances across requests while maintaining the same functionality.

• Before: Each translation request would create a new i18n instance

• After: i18n instances are cached and reused based on locale-namespace combinations

Mandatory Tasks (DO NOT REMOVE)

• I have self-reviewed the code (A decent size PR without self-review might be rejected).
• N/A - I have updated the developer docs in /docs if this PR makes changes that would require a documentation change. If N/A, write N/A here and check the checkbox.
• I confirm automated tests are in place that prove my fix is effective or that my feature works.

How should this be tested?

• Please use the latest Vercel preview and test please 🙏.

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2 Skipped Deployments

Name	Status	Preview	Comments	Updated (UTC)
cal	⬜️ Ignored (Inspect)			Dec 20, 2024 8:54pm
calcom-web-canary	⬜️ Ignored (Inspect)			Dec 20, 2024 8:54pm

[graphite-app]

Graphite Automations

"Add consumer team as reviewer" took an action on this PR • (12/20/24)

1 reviewer was added to this PR based on Keith Williams's automation.

[github-actions]

E2E results are ready!

• Workflow #42118.1 latest results