change(web): remove ES5 support

[ermshiperete]

The support of ES5 limits our ability to move forward (e.g. #8433) and presumably won't work with WASM. We can't use any newer language features since esbuild refuses to convert post-ES5 code to ES5. In a discussion with @jahorton it was decided that would remove the ES5 support for Keyman 18.