ENH: Migrate HigherOrderAccurateGradient content to ITK main; archive this repo

Author: hjmjohnson

CMakeLists.txt

@@ -0,0 +1,11 @@
+# Migrated to ITK main
+
+The `HigherOrderAccurateGradient` module has been ingested into the Insight Toolkit (ITK) main repository and is no longer maintained here.
+
+- **In-tree location:** [`Modules/Filtering/HigherOrderAccurateGradient`](https://github.com/InsightSoftwareConsortium/ITK/tree/main/Modules/Filtering/HigherOrderAccurateGradient)
+- **Ingest PR:** [InsightSoftwareConsortium/ITK#6242](https://github.com/InsightSoftwareConsortium/ITK/pull/6242)
+- **Merge commit:** [`970bba73bb`](https://github.com/InsightSoftwareConsortium/ITK/commit/970bba73bbd657ba8199d14af807a471da6d759a)
+
+Future development, bug fixes, and pull requests should target ITK main. This repository's history is preserved for `git blame` and archival reference.
+
+**Maintainer action:** please mark this repository **Archived** under repo Settings -> Danger Zone after merging this PR. See the [ITK ingestion strategy](https://github.com/InsightSoftwareConsortium/ITK/blob/main/Utilities/Maintenance/RemoteModuleIngest/INGESTION_STRATEGY.md) for context.