[CI-5972]: Remove deprecated JWT server-to-server authentication

[emanuelv-ens]

Description

This PR removes the deprecated JWT server-to-server authentication, the only remaining method of authentication being OAuth server-to-sever (using the client_credentials grant type).

Related Issue

#224 introduced OAuth server-to-server authentication.

Motivation and Context

JWT server-to-server authentication is deprecated and being actively removed in other Adobe products. This PR removes support for JWT server-to-server authentication.

How Has This Been Tested?

By running the tests in all the modules, using mvn test.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

[francoisledroff]

I'm surprised, your PR checks does not include the automated (GH actions powered) e2e checks.
do you know why ?
anyway we don't want to merge this now, as I heard we plan to support jwt a few more months... right ?