Remove head hydration workaround

[ghost]

Closes #1066

[benmccann]

@johnmuhl should this also update the peerDependency in package.json to set svelte 3.17.3 as the minimum version required?

[Conduitry]

I'm taking a look at this again now that we're committed to the next release containing breaking changes - and I'm bothered that removing this isn't breaking any tests, even though we aren't updating any of the tests to include hydratable: true in the bundler's compiler config.

[benmccann]

Every test already has hydratable: true in its rollup.config.js file, so I think that's to be expected

[Conduitry]

The version of Svelte used in the tests in this branch is ancient though -

sapper/package-lock.json

Line 5312 in fcdd67b

"version": "3.6.9",

- much older than the feature that this change relies on, which leads me to think this isn't being tested at all.

[benmccann]

Yeah, there's just one test for hydration and it does't test the head element:

sapper/test/apps/basics/test.ts

Line 170 in a5a421f

it('hydrates initial route', async () => {

Do we need to add a new test for it though? Svelte should have tests for head hydration and I'm not sure if we're doing anything special once the workaround is removed

[Conduitry]

The pkg.peerDependencies should at least be updated to reflect the new requirement. This probably doesn't need to be called out in the migration section in the docs. I guess I'm okay without a test for this, since Svelte should be already testing it.

[benmccann]

Ok, I just bumped the peerDependencies, so I'll go ahead and merge this one