Maintenance: Update README.md inline with v2

[phipag]

Summary

We need to update the README.md to stop mentioning that v2 is a preview version and also fix the badges:

Why is this needed?

This was forgotten to update when releasing v2.

Which area does this relate to?

Governance

Solution

No response

Acknowledgment

• This request meets Powertools for AWS Lambda (Java) Tenets
• Should this be considered in other Powertools for AWS Lambda languages? i.e. Python, TypeScript