MultiJson deprecation warning still emitted from Base#perform_request with multi_json >= 1.21 (PR #112 fix incomplete)

### Summary

`multi_json` 1.21.0 deprecated the legacy `MultiJson` constant in favor of `MultiJSON`. PR #112 handled this in the serializer (`Serializer::MultiJson`) by gating on `deprecated_gem_version_loaded?` and calling `::MultiJSON.parse` / `::MultiJSON.generate` for `multi_json >= 1.21.0`.

However, `Base#perform_request` still calls `::MultiJson.adapter` directly, without the same guard. With `multi_json >= 1.21.0` installed this emits, on the first JSON response of every process:

```
The MultiJson constant is deprecated and will be removed in v2.0. Use MultiJSON instead.
```

### Location

In `lib/elastic/transport/transport/base.rb`, inside `perform_request` (still present on `main`, commit `d192b57`):

```ruby
# Prevent Float value from automatically becoming BigDecimal when using Oj
load_options = {}
load_options[:mode] = :compat if ::MultiJson.adapter.to_s == "MultiJson::Adapters::Oj"
```

`::MultiJson.adapter` is a method call on the legacy constant, which routes through `multi_json`'s deprecation shim and triggers the warning. The serializer at `lib/elastic/transport/transport/serializer/multi_json.rb` is already guarded; this call site was missed.

### Environment

- `elastic-transport` 8.5.2 (and `main`)
- `multi_json` 1.21.1
- Ruby 4.0.1

### Reproduction

```ruby
require 'multi_json' # 1.21.x
Warning[:deprecated] = true
::MultiJson.adapter
# => warning: The MultiJson constant is deprecated and will be removed in v2.0. Use MultiJSON instead.
```

In a real app the warning appears at runtime on the first Elasticsearch response with a JSON `content-type` (once per process, since `multi_json` warns only once per key).

### Suggested fix

Gate the `adapter` lookup the same way the serializer does, e.g.:

```ruby
adapter = deprecated_gem_version_loaded? ? ::MultiJson.adapter : ::MultiJSON.adapter
load_options[:mode] = :compat if adapter.to_s.end_with?('Adapters::Oj')
```

(or expose the existing `deprecated_gem_version_loaded?` helper to `Base`, or compare against both `"MultiJson::Adapters::Oj"` and `"MultiJSON::Adapters::Oj"`).

---
_{Issue opened with the help of [Claude Code](https://claude.com/claude-code).}

Start in lib/elastic/transport/transport/base.rb at Base#perform_request and compare its adapter lookup with the guarded handling in lib/elastic/transport/transport/serializer/multi_json.rb. Reproduce with multi_json 1.21.x and Warning[:deprecated] enabled, then verify that a JSON response no longer emits the legacy MultiJson warning while the Oj compatibility behavior remains intact.

Written by the indexing model from the issue text.