Use calico_pool_blocksize from cluster when existing (#10516)

The blockSize attribute from Calico IPPool resources cannot be changed
once set [1]. Consequently, we use the one currently defined when
configuring the existing IPPool, avoiding upgrade errors by trying to
change it.

In particular, this can be useful when calico_pool_blocksize default
changes in kubespray, which would otherwise force users to add an
explicit setting to their inventories.

[1]: https://docs.tigera.io/calico/latest/reference/resources/ippool#spec