Support for OCPP 2.1

[knowack1]

[snabo1988]

Thanks for raising OCPP 2.1 support.

We agree that OCPP 2.1 is the future direction of the protocol.
Due to its architectural impact, we decided to intentionally defer implementation until the OCPP 1.6 MVP is finalized and released.

The decision and rationale are documented in ADR-004:
docs/adr/ADR-004-ocpp-21-support.md

This issue remains open as a roadmap and design discussion.
Design proposals and architectural feedback are welcome, while direct implementation PRs may be deferred until the post-MVP phase.