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>

modules/indexer/issues/internal/tests/tests.go

@@ -116,6 +116,16 @@ var cases = []*testIndexerCase{
 			assert.Equal(t, len(data), int(result.Total))
 		},
 	},
+	{
+		// Exercises the single-doc Index/Delete fast path in backends that have one (e.g. Elasticsearch).
+		Name: "single-doc index",
+		ExtraData: []*internal.IndexerData{
+			{ID: 999, Title: "solo-issue-marker"},
+		},
+		SearchOptions: &internal.SearchOptions{Keyword: "solo-issue-marker"},
+		ExpectedIDs:   []int64{999},
+		ExpectedTotal: 1,
+	},
 	{
 		Name: "Keyword",
 		ExtraData: []*internal.IndexerData{