docs: Add changelog for bt_mesh_adv_unref fix#22306

PavelVPV wants to merge 1 commit intonrfconnect:v2.9-branchnrfconnect/sdk-nrf:v2.9-branchfrom

PavelVPV:backport_bt_mesh_adv_unref_assert_fix_to_2.9.2_changelogPavelVPV/sdk-nrf:backport_bt_mesh_adv_unref_assert_fix_to_2.9.2_changelogCopy head branch name to clipboard