[Feature]: Add badge with published version

### Feature summary

In the README add badge with the latest published version from [pkg.go](https://pkg.go.dev/github.com/adyen/adyen-go-api-library/v21?tab=versions), like for example we do for the Java library
```
[![Maven Central](https://img.shields.io/maven-central/v/com.adyen/adyen-java-api-library)](https://central.sonatype.com/artifact/com.adyen/adyen-java-api-library/)
```

