avro

Remove 'default' from map and array examples (and add to enum). (#3046)

Default values are not valid for maps and arrays, and are only
valid for record fields (and enums, though that's a different story).

Having it in the examples is confusing, as people might
have expectations around it behaving in a certain way.

In the Java and Rust implementation, a default value listed here
will be packed into the custom props and attributes payloads, and
do not need to be of the same type.