…o unit tests (elastic#258142)

- Migrates ES deprecations coverage from flaky client integration tests
to focused unit tests.
- Adds unit coverage for the ES deprecations page/table (refresh,
counts, filters/search, pagination) and key deprecation types (default,
index settings, cluster settings, ML snapshots, indices reindex/readonly
actions, data streams migration).
- Removes the legacy `__jest__/client_integration/es_deprecations` specs
and harness.

- Closes elastic#241740
- Closes elastic#248431
- Closes elastic#248433
- Closes elastic#257356

- [Integration → unit test mapping
(gist)](https://gist.github.com/kapral18/223345de2e7b80156a8d82c94ca05098)

- [x] `node scripts/check_changes.ts`
- [x] `yarn test:jest
x-pack/platform/plugins/private/upgrade_assistant/public/application/components/es_deprecations`
- [x] `yarn test:type_check --project
x-pack/platform/plugins/private/upgrade_assistant/tsconfig.json`

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

* **Tests**
* Reorganized test infrastructure for Elasticsearch deprecations module,
consolidating mock data into centralized fixtures and refactoring
integration tests into focused unit tests for individual components and
scenarios.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

(cherry picked from commit bca01c1)