Update the `package.json` `engines` to contain a version range

[desrosj]

The engines field in the package.json file specifies the desired versions of NodeJS and (optionally) NPM.

The build server uses the node version specified here to determine which version of NodeJS to use when building WordPress after a commit. Until recently, this did not support version ranges (see ​https://make.wordpress.org/systems/2021/01/27/core-build-sync-node-version-selection/).

Specifying a range can make it more obvious to contributors that any version of Node >= the latest LTS version (currently 14.x) can be used, and not just the one specific version that has been specified previously.

trunk should be updated to specifify node >=14.15.0 and npm >=6.14.8.

[desrosj]

In 50185:

Build/Test Tools: Specify a version range within engines for node and npm.

This makes it more clear what versions of node and npm will successfully build WordPress.

Props dd32, mkaz.
Fixes #52455. See #51749.

[desrosj]

In 50186:

Build/Test Tools: Specify a version range within engines for node and npm.

This makes it more clear what versions of node and npm will successfully build WordPress.

Props dd32, mkaz.
Merges [50185] to the 5.6 branch.
Fixes #52455. See #51749.