Support json ^2

What does this MR do?

Supports json ^2 and makes the json dependency optional.

Simplifies the gitlab-ci.yml and updates the docker images.

This is not a major upgrade as this package does not derive any of the types for which the json encoding generics are now derived in the json package.

Related issues

N/A

Changes to public APIs

N/A

Author's checklist (required)

See CONTRIBUTING.md for the rationale behind these items:

• The commit history does not contain merges (use git rebase -i master if it does)
• Intermediate commits compile (use git rebase -i master if not)
• Newly added code follows the Platform code style
• Newly added code is documented
• If bugs have been solved, tests have been added
• Appropriate types have been used, especially in APIs
• If efficiency is part of the acceptance criteria of the issue, a benchmark is provided
• A changelog entry has been added if required. See CONTRIBUTING.md