## Summary
Two related cleanups, following the same pattern as #19916 and #19074.
### Dead feature flags
Drops four feature flags whose only references are the enum entry and
the generated GraphQL/SDK files:
- `IS_COMMAND_MENU_ITEM_ENABLED` — never read anywhere.
- `IS_DATASOURCE_MIGRATED` — already commented `@deprecated`. Zero
non-generated consumers.
- `IS_RICH_TEXT_V1_MIGRATED` — the 1-19 migration that gated it was
removed in #19074; the flag became dead at that point.
- `IS_CONNECTED_ACCOUNT_MIGRATED` — only read by the 1-21
`migrate-messaging-infrastructure-to-metadata` command as an
early-return guard, but the flag was never written anywhere in the
codebase, so the guard never fired (and that workspace command is now
removed entirely — see below).
Generated GraphQL/SDK schemas and the `workspace-entity-manager` test
mock are trimmed to match.
### 1-21 workspace commands
Same pattern as #19074 (which removed workspace commands ≤ 1.18). Twenty
is now on 2-5; the 1-21 workspace commands have long since run on every
active workspace and are dead code.
Removes:
- All 14 workspace commands under `upgrade-version-command/1-21/`
(compose-email menu item, key-value-pair index, datasource backfill,
message-thread backfill, dedup engine commands, select-all fixes, AI
response format migration, edit-layout label, drop messaging FKs, folder
parent-id migration, messaging-infra-to-metadata, navigation refactor,
message-thread label fix, search-menu-item label).
- The `1-21-upgrade-version-command.module.ts` registration and the
`V1_21_UpgradeVersionCommandModule` import from
`WorkspaceCommandProviderModule`.
**Kept** (intentionally): the 3 `1-21-instance-command-fast-*` files.
Unlike workspace commands (which mutate data), instance commands carry
**schema deltas** still required by current entity definitions
(`AddViewFieldGroupIdIndex`, `MigrateMessagingCalendarToCore`,
`AddEmailThreadWidgetType`). They remain registered in
`INSTANCE_COMMANDS` and `'1.21.0'` stays in `TWENTY_PREVIOUS_VERSIONS`.
They will fold away naturally on a future version bump when a
`CoreMigrationCheck`-style snapshot picks them up.
## Test plan
- [x] `npx nx typecheck twenty-shared`
- [x] `npx nx typecheck twenty-server`
- [x] `npx nx typecheck twenty-front`
- [x] `npx prettier --check` on the changed files
- [x] `npx oxlint` on the changed server files
- [x] `npx jest feature-flag` (4 suites, 20 tests pass)
- [x] `npx jest workspace-entity-manager` (1 suite, 5 tests pass)
