bump the node engine version

[Jimmy Wärting]

14 is currently the LTS version. what do you feel about bumping it to a higher version in order to use newer js features?

closure-compiler-npm/packages/google-closure-compiler/package.json

Lines 66 to 68 in 268f16b

 "engines": { 
   "node": ">=10" 
 } 

[Chad Killingsworth]

Yes it's OK to set the min supported version to Node 14. The CI jobs are already Node 14+.