Switch from Gson LinkedTreeMap to JDK LinkedHashMap as Map implementation

[Marcono1234]

Problem solved by the feature

Gson uses its internal LinkedTreeMap as Map implementation when a Map<String, ...> (or a raw Map) should be created. The reasons for this LinkedTreeMap seem to be that old JDK versions were vulnerable to denial-of-service attacks due to hash code collision. See previous discussions in #1992 (comment) (that also covers another Gson Map implementation which we had removed in the meantime) and #2152 (comment).

The problem is that Gson's LinkedTreeMap:

• Does not permit null keys
• Requires that keys must be Comparable

This can cause issues such as #1247, but also for less contrived cases where the user tries to add null or non-Comparable to the deserialized map, such as:

@SuppressWarnings("unchecked")
Map<Object, Object> map = new Gson().fromJson("{}", Map.class);
map.put(null, 1);

var map2 = new Gson().fromJson("{}", new TypeToken<Map<String, Integer>>() {});
map2.put(null, 1);

Note: I have added the java8 Issues related to making Java 8 the minimum supported version label because for Java >= 8 we can probably be relatively sure that LinkedTreeMap is not needed as denial-of-service protection, see JDK-8046170.
But we could also already make this change while Gson is still targeting Java 7 as minimum.

Feature description

We should consider not creating Gson's LinkedTreeMap in ConstructorConstructor anymore, but only JDK LinkedHashMap.
(And adjust unit tests which assert isInstanceof(LinkedTreeMap.class) and isNotInstanceof(LinkedTreeMap.class).)

Note: We can probably not completely remove LinkedTreeMap because:

• External users rely on it (even though they shouldn't because the class is internal); we could mark it as @Deprecated though
• JsonObject uses it, and especially its asMap method relies on LinkedTreeMap not permitting null keys and values
  Otherwise we would have to write a custom wrapper class which disallows null keys and values (similar to what we have for JsonArray#asList)

This would then:

• Resolve Confusing Exceptions #1247
• Obsolete Fix LinkedTreeMap being used for non-Comparable keys #2152

Alternatives / workarounds

• Try to fix at least some cases of ClassCastException due to LinkedTreeMap usage, see Fix LinkedTreeMap being used for non-Comparable keys #2152