refs https://github.com/TryGhost/Toolbox/issues/71
- Node 16 is now LTS so we're adding support in Ghost
- we're also bumping the minimum Node 14 version to 14.17.0 so we can merge
dependencies who have higher Node 14 requirements than current
- this commit adds Node 16 to the `package.json` engines and to CI
- also bumps all dependencies that needed new versions to add Node 16
support
refs 7e6800b2b8
- referenced commit deprecated `grunt main` in favor of `yarn main` but
this wasn't updated so we never installed dependencies for Admin,
which caused the build to break
- this commit switches to the new command
no issue
- by extracting it to a different workflow that uses the
`workflow_dispatch` trigger, we can manually run a build, but also
trigger it from Admin so we can always stay up-to-date
