Should be no need for mvCapture anymore with c++14 generalized lambdas

Originally posted by @maflcko in bitcoin/bitcoin#28735 (comment):

https://cirrus-ci.com/task/6337577544843264?logs=ci#L1948

  CXX      bitcoin_node-bitcoind.o
  CXX      init/bitcoin_node-bitcoin-node.o
  GEN      ipc/capnp/echo.capnp.h
  GEN      ipc/capnp/init.capnp.h
  CXX      ipc/capnp/libbitcoin_ipc_a-echo.capnp.o
  CXX      ipc/capnp/libbitcoin_ipc_a-init.capnp.o
  CXX      ipc/capnp/libbitcoin_ipc_a-echo.capnp.proxy-client.o
  CXX      ipc/capnp/libbitcoin_ipc_a-init.capnp.proxy-client.o
  CXX      ipc/capnp/libbitcoin_ipc_a-echo.capnp.proxy-server.o
  CXX      ipc/capnp/libbitcoin_ipc_a-init.capnp.proxy-server.o
  CXX      ipc/capnp/libbitcoin_ipc_a-echo.capnp.proxy-types.o
  CC       src/libsecp256k1_precomputed_la-precomputed_ecmult.lo
  CXX      ipc/capnp/libbitcoin_ipc_a-init.capnp.proxy-types.o
In file included from ipc/capnp/echo.capnp.proxy-server.c++:4:
/ci_container_base/depends/i686-pc-linux-gnu/include/mp/proxy-types.h:122:13: error: 'mvCapture<(lambda at /ci_container_base/depends/i686-pc-linux-gnu/include/mp/proxy-types.h:123:13), capnp::CallContext<ipc::capnp::messages::Echo::DestroyParams, ipc::capnp::messages::Echo::DestroyResults> &>' is deprecated: Use C++14 generalized captures instead. [-Werror,-Wdeprecated-declarations]
        kj::mvCapture(server_context.call_context,
            ^