whisper/whisperv6: fix PoW calculations to match the spec (#19330)

This PR fixes two issues in the PoW calculation of a Whisper envelope,
compared to the spec (see PoW Requirements):

- The pow is supposed to take the leading number of zeroes (i.e. most
  significant zeroes) and what it did was to take the number of trailing
  zeroes (i.e. least significant zeroes). It has been fixed to match what
  the spec and Parity does.
- The spec expects to use the size of the RLP encoded envelope, and it took
  something else, as described in #18070.