Update deprecated retrieve transaction implementation

[Sjustein]

The implementation of the 'Retrieve transaction' endpoint that is currently implemented in the sdk, is marked as deprecated in the sumup documentation (https://developer.sumup.com/api/transactions/get-deprecated). The new implementation seems to require the merchant code: https://developer.sumup.com/api/transactions/get . 2 questions:

• When is the old endpoint expected to be removed (and thus this sdk broken)?
• Is there a timeline for implementing the new endpoint instead?

[vendra95]

Hello, unfortunately SumUp doesn't seem very interested in updating this repository. As you can see, the last pull request is mine.

I have just started updating the library with the new changes. Please be patient, I hope to publish it as soon as possible, provided they accept the pull request quickly.