BC: drop support for `node`<v20.18

[jkowalleck]

node < 18 is no longer maintained, and will EOL soon.
see https://github.com/nodejs/release#release-schedule
see https://nodejs.org/en/about/previous-releases

lets drop support for node < v20.18 (last release in active state)

idea:

• Support node versions greater equal last active release -- so that all implemented features are included, and maintenance releases are compatible
• and so on

docs:

• update the readme and tell about the requirements. also mention that there are older versionswith lower requirements...

(this might enable more up-to-date developer experience, and updates in some [dev-]dependencies)

[jkowalleck]

or maybe dont drop that support,
but run/install dev-tools in a dedicated workspace -- like https://github.com/CycloneDX/cyclonedx-javascript-library/tree/main/tools