[CT-781] Bump manifest schema to v6 for v1.2

[jtcohen6]

v1.2 will include a schema change for manifest.json, with the addition of the grants: {} default config. We'll need to update, document v6, and add/declare backwards compatibility (per #5346):

dbt-core/core/dbt/contracts/graph/manifest.py

Lines 1138 to 1140 in d5608dc

	@classmethod
	def compatible_previous_versions(self):
	return [("manifest", 4)]

As I've said many many times before (in #4617 as elsewhere):

I'm interested in any small, tactical lifts we can make so that we're not bumping the manifest schema version in every single new minor version. At the same time, we're eager to provide other interfaces into manifest information, over which we have more control

Given that config can accept any values, it probably makes more sense to treat this in our JSONSchema as a dict[str, Any], rather than inferring its defaults from the node config classes.

The ability to declare backwards compatibility does remove this as an issue for most end users, but it remains true that we're creating unneeded manual work for metadata consumers.

This will also require updates to:

• https://github.com/dbt-labs/schemas.getdbt.com
• https://docs.getdbt.com/reference/artifacts/manifest-json

[leahwicz]

AC:

1. bump manifest to v6 in Core
2. pushing the change into the schemas repo
3. update docs

[jtcohen6]

Issue for updating docs: dbt-labs/docs.getdbt.com#1667

[emmyoop]

compatible_previous_versions was updated as part of #5027.

[jtcohen6]

I think this one is resolved! We have a separate issue to track the docs update