change map field default names to follow the parquet format spec

[Morgan279]

Which issue does this PR close?

Closes #6213 .

Rationale for this change

The MapBuilder uses the nonstandardized default names, which results in #6213 . Changing to parquet spec helps reduce confusion and provides users with a more standardized naming guide.

According to the parquet-format spec, the outer-most level should be a group that contains a single field named key_value for Map type:

The outer-most level must be a group annotated with MAP that contains a single field named key_value. The repetition of this level must be either optional or required and determines whether the map is nullable.
The middle level, named key_value, must be a repeated group with a key field for map keys and, optionally, a value field for map values. It must not contain any other values.

Changing the default map field names to match it not only complies with the parquet spec, but also aligns with pyarrow.

What changes are included in this PR?

Default value of the MapFieldNames

Are there any user-facing changes?

No(I think)

[tustvold]

Can you confirm pyarrow does follow this convention, the arrow spec has different guidance

https://github.com/apache/arrow-rs/blob/main/format%2FSchema.fbs#L133

I'm also rather wary of making this change as it will be highly disruptive, and for relatively limited benefit

Edit: in fact the linked issue shows pyarrow coercing when writing to parquet

import pyarrow as pa
import pyarrow.parquet as pq

pylist = [{"map_type":{'1':b"M"}}]
schema = pa.schema(
    [
        pa.field("map_type", pa.map_(pa.large_string(), pa.large_binary())),
    ]
)
table = pa.Table.from_pylist(pylist, schema=schema)

# table.schema
#
# map_type: map<large_string, large_binary>
#   child 0, entries: struct<key: large_string not null, value: large_binary> not null
#       child 0, key: large_string not null
#       child 1, value: large_binary

This boils down to a similar issue to #6733