Capnproto v2 support

[ryanofsky]

A Cap'n Proto v2 branch was announced https://capnproto.org/news/#whats-planned-for-20 and it would be good to make sure libmultiprocess is compatible with it.

@fanquake also flagged an issue in bitcoin/bitcoin#32669 with the v2 branch requiring GCC 14.2 or 15.1 that could affect us.

Useful steps to take would be to:

• Test building libmultiprocess & bitcoin with ENABLE_IPC=ON with the capnproto v2 branch.
• If there are any incompatibilities, fix them, maybe using the CAPNP_VERSION macro if needed to handle special cases.
• Add a CI job to the libmultiprocess repository to ensure compatibility going forward.