Revert "chore: formalize the default map field names to match default arrow spec (#10297)"

[alamb]

Which issue does this PR close?

N/A

cc @Jefffrey and @rluvaton

Rationale for this change

#10297 changed the default map field names from keys/values to key/value to match the Arrow spec. This is a good change to align with the spec, but it is a breaking change: it broke the DataFusion upgrade (apache/datafusion#24030) because data produced elsewhere (e.g. by other Arrow implementations) uses the old keys/values field names, causing schema mismatches such as:

InvalidArgumentError("Incorrect datatype for StructArray field \"metadata\", expected Map(\"entries\": non-null Struct(\"keys\": non-null Utf8, \"values\": Utf8), unsorted) got Map(\"entries\": non-null Struct(\"key\": non-null Utf8, \"value\": Utf8), unsorted)")

See discussion on #10297: #10297 (comment)

We should hold the field name change for the next breaking release to minimize downstream churn on a minor release, rather than ship it in a minor release.

What changes are included in this PR?

• Reverts b963ecf (chore: formalize the default map field names to match default arrow spec #10297), restoring the default map field names to keys/values (plural).
• Updates a test added by fix(arrow-json): validate map value nullability #10475 after chore: formalize the default map field names to match default arrow spec #10297 merged

Are these changes tested?

Existing tests.

Are there any user-facing changes?

Yes: this reverts the default MapFieldNames back to keys/values (as it was prior to #10297), rather than key/value. The intent is to reapply #10297 as part of the next breaking release.