Ignore partition fields that are dropped from the current-schema

[Fokko]

A second attempt to fix this (the first attempt in #11604 but that one was not safe).

When a field that's dropped that still part of an older partition spec, we currently get an error. See #11842

This will replace it with the UnknownType which will be read as a null. This will work fine, since when the evaluator encounters a null, it will assume that there is no value:

iceberg/api/src/main/java/org/apache/iceberg/expressions/Projections.java

Lines 204 to 207 in dbd7d1c

	if (parts == null) {
	// the predicate has no partition column
	return Expressions.alwaysTrue();
	}

I misused the UnknownType here, but I think it is better than returning an arbitrary type, like we do in the UnknownTransform:

iceberg/api/src/main/java/org/apache/iceberg/transforms/UnknownTransform.java

Lines 63 to 67 in dbd7d1c

	@Override
	public Type getResultType(Type type) {
	// the actual result type is not known
	return Types.StringType.get();
	}

Fixes #4563

[HonahX]

Hi @Fokko. I am working on a PR to add UnknownType support and just find that you've already done the most important part here : ).

I wonder if we also want to also update

iceberg/api/src/main/java/org/apache/iceberg/Schema.java

Lines 593 to 607 in 821aec3

	public static void checkCompatibility(Schema schema, int formatVersion) {
	// accumulate errors as a treemap to keep them in a reasonable order
	Map<Integer, String> problems = Maps.newTreeMap();
	// check each field's type and defaults
	for (NestedField field : schema.lazyIdToField().values()) {
	Integer minFormatVersion = MIN_FORMAT_VERSIONS.get(field.type().typeId());
	if (minFormatVersion != null && formatVersion < minFormatVersion) {
	problems.put(
	field.fieldId(),
	String.format(
	"Invalid type for %s: %s is not supported until v%s",
	schema.findColumnName(field.fieldId()), field.type(), minFormatVersion));
	}

in this PR to prevent users from adding UnknownType to v2 table schema? Is it ok to do this in a separate PR and ensure both goes before the next release?

Tracking issue for UnknownType: #11732