Bluetooth: CSIP: Remove bt_csip_set_member_get_sirk

[Thalley]

Is your enhancement proposal related to a problem? Please describe.
The bt_csip_set_member_get_sirk function is just a subset of bt_csip_set_member_get_info introduced in #86826 and can/should thus be removed and uses of it replaced with bt_csip_set_member_get_info

Describe the solution you'd like
Remove the bt_csip_set_member_get_sirk function.
Places that use it in the Zephyr tree should switch to bt_csip_set_member_get_info.
Since this is an unstable API, we can remove without the deprecation process, but an entry should be added in the migration guide.

Describe alternatives you've considered
N/A

Additional context
N/A