Remove dev feature flag from POST /extension-requests API

[samarpan1738]

Issue Description

This API has been live on prod for a long time but the feature flag has not been removed.
Remove the dev FF from backend and all other repos where its being used.

Reasons for adding dev FF

1. Create multiple extension requests - PR link

Usages in other repos

1. mobile-app - FF not being used at all 🟥 . Communicated to Shreya about this. No breaking change except some error message changes
2. website-my - All usages are using dev feature flag ✅
3. website-api-contracts - All usages are using dev feature flag ✅

This is a subtask of #2075

Expected Behavior

Current Behavior

Screenshots

NA

Reproducibility

• This issue is reproducible
• This issue is not reproducible

Steps to Reproduce

NA

Severity/Priority

• Critical
• High
• Medium
• Low

Additional Information

Checklist

• I have read and followed the project's code of conduct.
• I have searched for similar issues before creating this one.
• I have provided all the necessary information to understand and reproduce the issue.
• I am willing to contribute to the resolution of this issue.