[FR] Improvement: Change updateChildren parameter to Map<String, ?>

**Is your feature request related to a problem? Please describe.**
Various builders (e.g., `Map.of`) and language features (e.g., Groovy and Kotlin inline maps) create `Map` instances whose `V` bound is more strict than `Object`. Currently, the `DatabaseRef#updateChildren[Async]` interface won't accept these instances because it is defined as accepting `Map`.

**Describe the solution you'd like**
Since these method consume the map and do not modify it, they could be defined as `Map` to accept a map with any generic value type with no backwards incompatibility or other code changes.

**Describe alternatives you've considered**
Otherwise, passing in maps usually requires an explicit type declaration instead of `var` or an inline builder.

DatabaseRef#updateChildren と updateChildrenAsync のエントリポイントを検索し、それらの宣言と関連するテストを確認します。メソッドが既存の更新動作を変更せずに Map を受け入れることを確認し、関連する Java テストスイートを実行します。

索引モデルが issue の本文から書いたものです。