Maintenance: Update discord badge

[hjgraca]

Why is this needed?

Currently discord badge url is not working and we are seeing a broken image in the repository readme.

Which area does this relate to?

Solution

Acknowledgment

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

[powertools-for-aws-oss-automation]

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[github-actions]

This is now released under 3.21.0 version!