refactor(upgrade): move indexDataPlatformsStep and migrateHomePageLinksStep to non-blocking system update

[Dutt23]

## Why

`IndexDataPlatformsStep` and `MigrateHomePageLinksStep` were blocking GMS startup
unnecessarily — running sequentially before GMS could serve traffic, despite neither
being critical to request serving.

`IndexDataPlatformsStep` also had a correctness bug: the original boot step fetched
only the first 1000 data platform URNs and stopped, silently skipping any beyond that
page. Any deployment with >1000 data platforms would have an incomplete search index
after running this migration.

## What

1. **Moves both steps** from `BootstrapManagerFactory` (blocking, sequential) to the
   `datahub-upgrade` `NonBlockingSystemUpgrade` pattern so GMS serves traffic
   immediately while these steps run out-of-band.

2. **Fixes pagination bug** in `IndexDataPlatformsStep`: the new implementation
   iterates pages of 1000 URNs until `start >= listResult.getTotal()`, ensuring all
   data platforms are indexed regardless of count.

### Changes

- Added `IndexDataPlatforms` / `IndexDataPlatformsStep` under `system/dataplatforms/`
  with paginated `listUrns` loop (`BATCH_SIZE=1000`, iterates all pages)
- Added `MigrateHomePageLinks` / `MigrateHomePageLinksStep` under `system/homepagelinks/`
- `showHomePageRedesign` flag gate preserved at wrapper construction time — step is a
  no-op when flag is off
- `STEP_ID` / `VERSION` match original boot step constants for idempotency (existing
  deployments that already ran the old step will skip)
- Added config flags (default `true`):
  ```
  systemUpdate.indexDataPlatforms.enabled
  systemUpdate.migrateHomePageLinks.enabled
  ```
- Removed dead source files from `metadata-service/factories/boot/steps/`
- Added unit tests for both steps

## Bug Fix

| | Old (boot step) | New (system-update step) |
|---|---|---|
| Fetch strategy | `listUrns(start=0, count=1000)` once | Paginated loop until `start >= total` |
| Data platforms >1000 | ❌ Silently truncated | ✅ Fully indexed |

## Impact

GMS startup no longer blocks on these steps. Data platforms >1000 are now fully indexed
after migration. Follows the same pattern established in #17691.

> **Note:** To disable either step, set the corresponding `systemUpdate.*.enabled`
> flag to `false`.

[codecov]

Codecov Report

❌ Patch coverage is 88.34951% with 24 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...e/system/dataplatforms/IndexDataPlatformsStep.java	86.36%	10 Missing and 5 partials ⚠️
...system/homepagelinks/MigrateHomePageLinksStep.java	93.50%	3 Missing and 2 partials ⚠️
...grade/system/dataplatforms/IndexDataPlatforms.java	75.00%	1 Missing and 1 partial ⚠️
...ade/system/homepagelinks/MigrateHomePageLinks.java	75.00%	1 Missing and 1 partial ⚠️

📢 Thoughts on this report? Let us know!

[codecov]

Bundle Report

Changes will increase total bundle size by 19.97kB (0.09%) ⬆️. This is within the configured threshold ✅

Detailed changes

Bundle name	Size	Change
datahub-react-web-esm	23.4MB	19.97kB (0.09%) ⬆️

Affected Assets, Files, and Routes:

view changes for bundle: datahub-react-web-esm

Assets Changed:

Asset Name	Size Change	Total Size	Change (%)
assets/index-*.js	6.72kB	8.82MB	0.08%
assets/en-*.js	13.24kB	126.46kB	11.7% ⚠️