Wrap ICU ucol_open().
Hide details of supporting older ICU versions in a wrapper
function. The current code only needs to handle
icu_set_collation_attributes(), but a subsequent commit will add
additional version-specific code.
Discussion: https://postgr.es/m/
7ee414ad-deb5-1144-8a0e-
b34ae3b71cd5@enterprisedb.com
Reviewed-by: Peter Eisentraut