[cdk-responsive-email-template] Upgrade to CDK v2; Upgrade dependencies

[e-pavlica]

Upgrades @cloudcomponents/cdk-responsive-email-template to use CDK v2.

I originally forked this because of a vulnerability alert in the html-minifier dependency; as it appears that package is no longer maintained I switched it to html-minifier-next. This does not actually solve the vulnerable dependency issue, as mjml also relies on html-minifier. I'm reticent to use an alpha tag, but upgrading mjml to >=5.0.0-alpha.0 would resolve the issues raised by npm audit for this package. If the maintainers here think that is appropriate change I'll happily apply that upgrade as well.

I don't have a lot of experience using yarn / lerna so please let me know if I didn't do something right with this upgrade process.