Upgrade the minimal required node release to 14.x, since Node 12 has
been end-of-life since `2022-04-01`. This allows us to upgrade our
TypeScript `target` from `ES2019` to `ES2020` (native support for
nullish-coalescing and optional chaining, among other things).

Removed the `node12` entries from the various test matrices and stop
publishing `node12` tags for `jsii/superchain`.

BREAKING CHANGE: Beginning with this release, jsii packages no longer
support node 12. Users should migrate to a supported node release (14.x,
16.x, or 18.x).

# Conflicts:
#	packages/@jsii/check-node/package.json
#	packages/@jsii/dotnet-runtime-test/package.json
#	packages/@jsii/dotnet-runtime/package.json
#	packages/@jsii/go-runtime/package.json
#	packages/@jsii/integ-test/package.json
#	packages/@jsii/java-runtime/package.json
#	packages/@jsii/kernel/package.json
#	packages/@jsii/runtime/package.json
#	packages/@jsii/spec/package.json
#	packages/@scope/jsii-calc-base-of-base/package.json
#	packages/@scope/jsii-calc-base/package.json
#	packages/@scope/jsii-calc-lib/package.json
#	packages/codemaker/package.json
#	packages/jsii-calc/package.json
#	packages/jsii-config/package.json
#	packages/jsii-diff/package.json
#	packages/jsii-pacmak/package.json
#	packages/jsii-reflect/package.json
#	packages/jsii-rosetta/package.json
#	packages/jsii/lib/compiler.ts
#	packages/jsii/package.json
#	packages/oo-ascii-tree/package.json
#	tools/jsii-compliance/package.json
#	yarn.lock