chore(deps): deprecate NodeJS pre 18.*

[jusiskin]

Problem

The mainline branch has an unreleased change that raises the minimum required version of NodeJS from 14.15.0 → 18.0.0 for installing the aws-rfdk NPM package. This becomes a breaking change for RFDK users and they would need to perform a NodeJS migration in order to adopt the new version.

Solution

Instead of an immediate breaking change, this PR deprecates support for NodeJS versions earlier to 18.

• Users will resume being able to install the next RFDK release on NodeJS >= 14.15.0
• The RFDK will emit a warning when imported about the deprecation
• The RFDK 1.3.0 upgrade documentation and README.md have been updated to reflect this adjustment

Testing

Used NodeJS 14.21.3 to deploy the All-In-AWS-Infrastructure-Basic TypeScript example successfully.

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