CDK-238 deprecate partition key

Note that this is needed to turn off our fail on warnings default setting. Deprecating PartitionKey causes a warning even with @SuppressWarnings, and we can't avoid its usage since PartitionStrategy has methods that return a PartitionKey, and for compatibility it has to be the deprecated one (not the SPI one).

Once 0.15.0 is out we can remove PartitionKey and this change in maven-compiler-plugin.