[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.

Suche nach den Einstiegspunkten DatabaseRef#updateChildren und updateChildrenAsync und untersuche anschließend deren Deklarationen sowie alle zugehörigen Tests. Bestätige, dass die Methoden Map akzeptieren, ohne ihr bestehendes Update-Verhalten zu ändern, und führe die relevante Java-Testsuite aus.

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