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>

.github/workflows/pull-db-tests.yml

@@ -102,9 +102,10 @@ jobs:
     runs-on: ubuntu-latest
     services:
       elasticsearch:
-        image: elasticsearch:7.5.0
+        image: docker.elastic.co/elasticsearch/elasticsearch:8.19.14
         env:
           discovery.type: single-node
+          xpack.security.enabled: false
         ports:
           - "9200:9200"
       meilisearch:
@@ -180,9 +181,10 @@ jobs:
         options: >-
           --mount type=tmpfs,destination=/bitnami/mysql/data
       elasticsearch:
-        image: elasticsearch:7.5.0
+        image: docker.elastic.co/elasticsearch/elasticsearch:8.19.14
         env:
           discovery.type: single-node
+          xpack.security.enabled: false
         ports:
           - "9200:9200"
       smtpimap: