Mark send RPC experimental
https://github.com/bitcoin/bitcoin/pull/19969/commits/efc9b85e6f4aa431d308089874a18f0bbdcdd0fd
[rpc] send: various touch-ups
https://github.com/bitcoin/bitcoin/pull/19969/commits/d813d26f06248aaa7be3c698c87939cc777fafd0
The other two commits are not relevant. The second commit of this PR fixed a bug related to RBF, which did not affect us.
This is a backport of core#19969