Upgrade the minimum node version for the CDKv2 packages from Node 10 to Node 14.

This was previously introduced by #11964 and reverted by #12704, due to incompatibilities with the jsii/superchain image. The jsii/superchain image now vends a Node14-specific/compatible version, so we can re-introduce this change.

The current applicable tag for the Node14 version of the superchain image is jsii/superchain:1-buster-slim-node14 (example: https://hub.docker.com/layers/jsii/superchain/1-buster-slim-node14/images/sha256-4074257429752108ebcf8dab91f66edc15aecb7a6a9163c5b2e2ed5d9dd7b323?context=explore).

This change should be done in tandem with the corresponding change in the CDK build + deploy infrastructure. See (private) linked issue for more details.