chore: update readme and workflow to require go 1.21

[livio-a]

Definition of Ready

• [x] I am happy with the code
• [ ] Short description of the feature/issue is added in the pr description
• [ ] PR is linked to the corresponding user story
• [ ] Acceptance criteria are met
• [ ] All open todos and follow ups are defined in a new ticket and justified
• [ ] Deviations from the acceptance criteria and design are agreed with the PO and documented.
• [ ] No debug or dead code
• [ ] My code has no repetitions
• [ ] Critical parts are tested automatically
• [ ] Where possible E2E tests are implemented
• [ ] Documentation/examples are up-to-date
• [ ] All non-functional requirements are met
• [ ] Functionality of the acceptance criteria is checked manually on the dev system.

[github-actions[bot]]

:tada: This PR is included in version 2.3.0 :tada:

The release is available on GitHub release

Your semantic-release bot :package::rocket: