This promotes and expands our existing optional migration.
Based on usage statistics from several instances, see:
#764
activities_hosts is now retained after all since it’s essential
for the "instance" query parameter of *oma’s public timeline to
reliably work in a reasonable amount of time. (Although akkoma-fe has
no support for this feature and apparently barely anyone uses it.)
activities_actor_index was already dropped before in
20221211234352_remove_unused_indices; no need to drop it again.
Birthday indices were introduced in pleroma starting with
20220116183110_add_birthday_to_users which is past the
last common migration 20210416051708.
