Replace olivere/elastic with REST API client, add OpenSearch support (#37411)

Drops `github.com/olivere/elastic/v7` (unmaintained) and replaces it
with a small in-house wrapper that speaks the Elasticsearch REST API
directly via `net/http`. The subset used by Gitea (`_cluster/health`,
`_bulk`, `_doc`, `_delete_by_query`, `_refresh`, `_search`, `HEAD`/`PUT`
index) is stable across the targeted servers, so no client library is
needed.

**Targets tested**
- Elasticsearch 7, 8, 9
- OpenSearch 1, 2, 3

**Why not `go-elasticsearch`?**
The official client enforces an `X-Elastic-Product` server-identity
check that OpenSearch deliberately fails, which would force shipping a
transport shim to defeat it. Going direct over `net/http` removes that
fight along with several MB of transitive deps (`elastic-transport-go`,
`go.opentelemetry.io/otel{,/metric,/trace}`, `auto/sdk`, `easyjson`,
`intern`, `logr`, `stdr`).

Replaces: #30755
Fixes: https://github.com/go-gitea/gitea/issues/30752

---
This PR was written with the help of Claude Opus 4.7

---------

Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

custom/conf/app.example.ini

@@ -1524,7 +1524,7 @@ LEVEL = Info
 ;; Issue Indexer settings
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;;
-;; Issue indexer type, currently support: bleve, db, elasticsearch or meilisearch default is bleve
+;; Issue indexer type, currently support: bleve, db, elasticsearch (also compatible with OpenSearch) or meilisearch default is bleve
 ;ISSUE_INDEXER_TYPE = bleve
 ;;
 ;; Issue indexer storage path, available when ISSUE_INDEXER_TYPE is bleve
@@ -1551,7 +1551,7 @@ LEVEL = Info
 ;; If empty then it defaults to `sources` only, as if you'd like to disable fully please see REPO_INDEXER_ENABLED.
 ;REPO_INDEXER_REPO_TYPES = sources,forks,mirrors,templates
 ;;
-;; Code search engine type, could be `bleve` or `elasticsearch`.
+;; Code search engine type, could be `bleve` or `elasticsearch` (also compatible with OpenSearch).
 ;REPO_INDEXER_TYPE = bleve
 ;;
 ;; Index file used for code search. available when `REPO_INDEXER_TYPE` is bleve