Bump the minimum required versions of Node and npm

[donnapep]

As per this post, WordPress Core and Gutenberg are on Node >=20.10.0 and npm >=10.2.3. We should update Sensei to match.

As part of this work, we should remove the hard-coded Node requirement from the GitHub workflows that were added in #7720.

Note that upgrading Node was already started in https://github.com/Automattic/sensei-security/pull/15.

[donnapep]

Also linking here for consideration / discussion the idea of eliminating the .nvmrc file:

In order to bring down technical debt, whenever a developer works on a site and rebuilds the assets, they should also update the packages and the engines entry in the package.json file, and try to fix any incompatibilities. It should be much easier to upgrade from Node.js 16 to 18 and so on than from Node.js 16 to 24 when the current packages are likely to first break.

We feel that including an .nvmrc file would be counter-productive to this goal of "forcing" maintainers to upgrade the packages and the Node.js version.

It could be contributing to the situation we find ourselves in now with outdated dependencies.