Bump to C++20 prevents usage on Ubuntu 20.04 with Node.js <= 20

[segevfiner]

Node.js <= 20 minimum OS is Ubuntu 20.04, but this package has moved to C++20 which is unsupported on Ubuntu 20.04, which leads to tree-sitter/workflows#16

That is, the official workflows to build grammars fail on Ubuntu 20.04 due to this if node-tree-sitter is upgrade to 0.22.0.

[ObserverOfTime]

The bump came after the breakage.
As far as I can tell, Node 23 uses C++20 and node-gyp uses that instead of the running version for whatever reason.

[segevfiner]

That's prebuildify defaulting to the latest Node.js version for building node-api packages, we will also need to add --target to its invocation with an older version like 20.18.0 that uses C++17. (And also tree-sitter itself not requiring C++20 for this to work).