docs(README.md): update min version of node to be 18

This project's package.json has specified version 18 as the min node version for a while now.  This commit just updates the README.md to reflect that.

Signed-off-by: Phil Adams <phil_adams@us.ibm.com>

README.md

@@ -13,8 +13,8 @@ This project contains core functionality required by Node.js code generated by t
 ```
 
 ## Prerequisites
-- Node.js version 14 or newer
-- Note that v14 is technically *supported* but not officially *guaranteed* as we only test on v16 and higher
+- Node.js version 18 or newer
+- Note that v18 is technically *supported* but not officially *guaranteed* as we only test on v16 and higher
 
 ## Usage
 This package exports a single object containing a number of modules as top level properties.