Deprecate old relations completely (#12482)

# What

Fully deprecate old relations because we have one bug tied to it and it
make the codebase complex

# How I've made this PR:
1. remove metadata datasource (we only keep 'core') => this was causing
extra complexity in the refactor + flaky reset
2. merge dev and demo datasets => as I needed to update the tests which
is very painful, I don't want to do it twice
3. remove all code tied to RELATION_METADATA /
relation-metadata.resolver, or anything tied to the old relation system
4. Remove ONE_TO_ONE and MANY_TO_MANY that are not supported
5. fix impacts on the different areas : see functional testing below 

# Functional testing

## Functional testing from the front-end:
1. Database Reset ✅
2. Sign In ✅
3. Workspace sign-up ✅
5. Browsing table / kanban / show ✅
6. Assigning a record in a one to many / in a many to one ✅
7. Deleting a record involved in a relation ✅ => broken but not tied to
this PR
8. "Add new" from relation picker ✅ => broken but not tied to this PR
9. Creating a Task / Note, Updating a Task / Note relations, Deleting a
Task / Note (from table, show page, right drawer) ✅ => broken but not
tied to this PR
10. creating a relation from settings (custom / standard x oneToMany /
manyToOne) ✅
11. updating a relation from settings should not be possible ✅
12. deleting a relation from settings (custom / standard x oneToMany /
manyToOne) ✅
13. Make sure timeline activity still work (relation were involved
there), espacially with Task / Note => to be double checked ✅ => Cannot
convert undefined or null to object
14. Workspace deletion / User deletion ✅ 
15. CSV Import should keep working ✅ 
16. Permissions: I have tested without permissions V2 as it's still hard
to test v2 work and it's not in prod yet ✅
17. Workflows global test ✅ 

## From the API:
1. Review open-api documentation (REST) ✅ 
2. Make sure REST Api are still able to fetch relations ==> won't do, we
have a coupling Get/Update/Create there, this requires refactoring
3. Make sure REST Api is still able to update / remove relation => won't
do same

## Automated tests
1. lint + typescript ✅
2. front unit tests: ✅
3. server unit tests 2 ✅
4. front stories: ✅
5. server integration: ✅
6. chromatic check : expected 0
7. e2e check : expected no more that current failures

## Remove // Todos
1. All are captured by functional tests above, nothing additional to do

## (Un)related regressions
1. Table loading state is not working anymore, we see the empty state
before table content
2. Filtering by Creator Tim Ap return empty results
3. Not possible to add Tasks / Notes / Files from show page

# Result

## New seeds that can be easily extended
<img width="1920" alt="image"
src="https://github.com/user-attachments/assets/d290d130-2a5f-44e6-b419-7e42a89eec4b"
/>

## -5k lines of code
## No more 'metadata' dataSource (we only have 'core)
## No more relationMetadata (I haven't drop the table yet it's not
referenced in the code anymore)
## We are ready to fix the 6 months lag between current API results and
our mocked tests
## No more bug on relation creation / deletion

---------

Co-authored-by: Weiko <corentin@twenty.com>
Co-authored-by: Félix Malfait <felix@twenty.com>

packages/twenty-website/src/content/developers/backend-development/server-commands.mdx

@@ -48,7 +48,7 @@ npx nx run twenty-server:database:reset
 #### For objects in Core/Metadata schemas (TypeORM)
 
 ```bash
-npx nx run twenty-server:typeorm migration:generate src/database/typeorm/metadata/migrations/nameOfYourMigration -d src/database/typeorm/metadata/metadata.datasource.ts # replace by core data source if necessary
+npx nx run twenty-server:typeorm migration:generate src/database/typeorm/core/migrations/nameOfYourMigration -d src/database/typeorm/core/core.datasource.ts
 ```
 
 #### For Workspace objects