[backport#14890] rpc: Avoid creating non-standard raw transactions

Summary:
rpc: Avoid creating non-standard raw transactions (MarcoFalke)

Pull request description:

Multiple OP_RETURN outputs in a transaction are not standard and unlikely to be relayed, so avoid creating them.

Apart from that, the logic was broken in that it duplicated the same hex-data for each data output: Closes #14868.

https://github.com/bitcoin/bitcoin/pull/14890/commits/fa4c8679ed94f215ce895938f7c3c169a2ce101e

Backport of Core PR14890

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7315