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