Align default channel groupings with GA4 docs

[erikverheij]

Description & motivation

Align default channel groupings with GA4 docs. The following things have been changed:

• Order of statements aligned with order in GA4 docs.
• Refactored test: Included more test cases and combined input with the expected channel for easier readability.
• Added campaign param to default_channel_grouping() method as it was needed for some mappings.
• Included GA4 descriptions to make it easier to maintain.
• Updated ga4_source_categories.csv based on PDF download from https://support.google.com/analytics/answer/9756891?hl=en#list
• Removed the reference to https://github.com/llooker/ga_four_block_dev as I don't think their implementation is used anymore.

Also see issue #273.

Checklist

• [ ✅] I have verified that these changes work locally
• [ not applicable] I have updated the README.md (if applicable)
• [✅] I have added tests & descriptions to my models (and macros if applicable)
• [✅ ] I have run dbt test and python -m pytest . to validate existing tests

[erikverheij]

Great! Thanks for reviewing & merging it so quickly!