DiffableUtils.diff should return an Immutable Map when used with a JDK Map

[grcevski]

Description

When using the DiffableUtils.diff with a vanilla OpenJDK Map, instead of the legacy ImmutableOpenMap interface, we return a mutable map (generic HashMap), which isn't the expected behaviour of the API:

elasticsearch/server/src/main/java/org/elasticsearch/cluster/DiffableUtils.java

Line 85 in 45c56b2

public static <K, T extends Diffable<T>> MapDiff<K, T, Map<K, T>> diff(

We should wrap the map returned by the diff API in an unmodifiable version.

[elasticmachine]

Pinging @elastic/es-core-infra (Team:Core/Infra)