Bump mrml from 3.1.5 to 4.0.0 in /native/mjml_nif

[moomerman]

Version 4.0.0 fixes a bug if you use phoenix function components to generate MJML and have heex annotations enabled by default.

Essentially the heex annotations add comments outside the tag and this was breaking the pareser. The bug has been fixed upstream.

See https://github.com/jdrouet/mrml/issues/409 for more info.

[moomerman]

~~I tried to update to the latest MJML release but the tests are giving the confusing error:~~

error: failed to select a version for `mrml`.
    ... required by package `mjml_nif v4.0.0 (/home/runner/work/mjml_nif/mjml_nif/native/mjml_nif)`
versions that meet the requirements `^4.0.0` are: 4.0.0

~~It looks like it is looking for 4.0.0 and found a 4.0.0 release but couldn't use it~~

[Edit: I spotted that it needed to be "4.0" not "4.0.0" 🤦🏼 ]