chore: remove IS_DASHBOARD_V2_ENABLED feature flag (#19079)

## Summary
- Remove the `IS_DASHBOARD_V2_ENABLED` feature flag from the codebase
- Dashboard V2 features (gauge charts, line charts, pie charts) are now
always enabled
- Remove the validator gate that blocked gauge chart creation/update
without the flag
- Clean up all related code: seed data, dev-seeder service, widget
seeds, and test mocks

packages/twenty-server/src/engine/twenty-orm/entity-manager/workspace-entity-manager.spec.ts

@@ -238,7 +238,6 @@ describe('WorkspaceEntityManager', () => {
         IS_RECORD_PAGE_LAYOUT_EDITING_ENABLED: false,
         IS_PUBLIC_DOMAIN_ENABLED: false,
         IS_EMAILING_DOMAIN_ENABLED: false,
-        IS_DASHBOARD_V2_ENABLED: false,
         IS_ROW_LEVEL_PERMISSION_PREDICATES_ENABLED: false,
         IS_JUNCTION_RELATIONS_ENABLED: false,
         IS_COMMAND_MENU_ITEM_ENABLED: false,
@@ -277,7 +276,6 @@ describe('WorkspaceEntityManager', () => {
         IS_APPLICATION_ENABLED: false,
         IS_PUBLIC_DOMAIN_ENABLED: false,
         IS_EMAILING_DOMAIN_ENABLED: false,
-        IS_DASHBOARD_V2_ENABLED: false,
         IS_ROW_LEVEL_PERMISSION_PREDICATES_ENABLED: false,
       },
       permissionsPerRoleId: {},