deprecate materialization overrides from imported packages (#9971)

[MichelleArk]

Same changes as 1.7 backport: #9998

Backport of #9956 to 1.6

Solution

The easiest way to explain & review this PR is commit-by-commit:

• 4f5bbd5 and ad570bb: backport deprecation warning when materialization from package overrides built-in materialization (original PR: deprecate materialization overrides from imported packages #9971)
• 6e99dc4 simply updates the documentation link in the deprecation warning
• 455d817 brings the UserConfig -> ProjectFlags capability to 1.7 (original PR: Move UserConfig to dbt_project.yml and rename to ProjectFlags #9289)
• 9a1ae15 brings the ability to set project-only flags via ProjectFlags (original PR: Feature/source freshness hooks #9366)
• 1c40830 finally adds the require_explicit_package_overrides_for_builtin_materializations flag to ProjectFlags

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX
• This PR includes type annotations for new and modified functions

No breaking interface changes. However, a deprecation warning will now be emitted for projects with flags defined in profiles.yml instead of dbt_project.yml. This PR introduces the capability to define flags in dbt_project.yml so I don't see an issue there.

[MichelleArk]

Confirmed same 🎩 scenarios work as expected as tested for the related 1.7 backport (#9998 (comment))

• dbt run passes as usual
• dbt run when profiles.yml specifies config (but emits deprecation warning)
• dbt run when dbt_project.yml specifies flags + respects their configurations as expected
• custom materialization from root project works as expected
• added a 'default' materialization override for view in an installed package -> saw deprecation warning
• ^ + setting require_explicit_package_overrides_for_builtin_materializations: true -> no deprecation warning and confirmed materialization override not reached
• require_explicit_package_overrides_for_builtin_materializations: false` -> deprecation warning, materialization override occurred