chore(server): bump current version to 2.0.0 and add 2.1.0 as next (#19907)

## Summary

We are releasing Twenty v2.0. This PR sets up the
upgrade-version-command machinery for the new release line:

- Move `1.23.0` into `TWENTY_PREVIOUS_VERSIONS` (it just shipped)
- Set `TWENTY_CURRENT_VERSION` to `2.0.0` (no specific upgrade commands
— this is just the major version cut)
- Set `TWENTY_NEXT_VERSIONS` to `['2.1.0']` so future PRs that
previously would have targeted `1.24.0` now target `2.1.0`
- Add empty `V2_0_UpgradeVersionCommandModule` and
`V2_1_UpgradeVersionCommandModule` and wire them into
`WorkspaceCommandProviderModule`
- Refresh the `InstanceCommandGenerationService` snapshots to reflect
the new current version (`2.0.0` / `2-0-` slug)

The `2-0/` directory is intentionally empty — there are no specific
upgrade commands for the v2.0 cut. New upgrade commands authored after
this merges should land in `2-1/` (or be generated against `--version
2.1.0`).

## Test plan

- [x] `npx jest` on the impacted upgrade test files
(`upgrade-sequence-reader`, `upgrade-command-registry`,
`instance-command-generation`) passes (41 tests, 8 snapshots)
- [x] `prettier --check` and `oxlint` clean on touched files
- [ ] Manual: open `nx run twenty-server:command -- upgrade --dry-run`
against a local stack with workspaces still on `1.23.0` and confirm the
sequence is computed without errors

Made with [Cursor](https://cursor.com)

packages/twenty-server/src/engine/core-modules/upgrade/constants/twenty-current-version.constant.ts

@@ -1 +1 @@
-export const TWENTY_CURRENT_VERSION = '1.23.0' as const;
+export const TWENTY_CURRENT_VERSION = '2.0.0' as const;

packages/twenty-server/src/engine/core-modules/upgrade/constants/twenty-next-versions.constant.ts

@@ -1 +1 @@
-export const TWENTY_NEXT_VERSIONS = [] as const;
+export const TWENTY_NEXT_VERSIONS = ['2.1.0'] as const;

packages/twenty-server/src/engine/core-modules/upgrade/constants/twenty-previous-versions.constant.ts

@@ -1 +1 @@
-export const TWENTY_PREVIOUS_VERSIONS = ['1.21.0', '1.22.0'] as const;
+export const TWENTY_PREVIOUS_VERSIONS = ['1.21.0', '1.22.0', '1.23.0'] as const;