fix(gatsby): Remove default support for non ESM browsers

[marvinjude]

Description

Drop default support for non-esm browsers in v5

[sc-52423]

[imjoshin]

Can we add tests to packages/gatsby/src/utils/__tests__/browserslist.js ?

[imjoshin]

I'm not sure we'll ever get to this point in the tests. Given that CFLAGS are environmental set by the compiler, they won't exist in the tests. I could be wrong, but either way, we won't ever be testing both cases.

Can we add a _CFLAGS_ mock to the setup/teardown? This might need some tweaking, but something like:

let originalCflags
beforeEach(() => {
  originalCflags = _CFLAGS_
  
  # TODO set up mock vars, set them in tests, etc
  _CFLAGS_ = {...}
})

afterEach(() => {
  _CFLAGS_ = originalCflags
})

[marvinjude]

We do pass in CFLAGS flags to test with babel-preset-gatsby-package.

My thinking is that we'll eventually remove the else block since we'll never hit that when the compile flags becomes 5

cc - @wardpeet

[imjoshin]

Ahh ok. I won't push too hard on it because the logic is simple, but I'd prefer to have full logic coverage even before the Gatsby 5 launch.