fix: make drop_uniqueness_constraint_on_phone idempotent (#1817)

Author: kangmingtay

migrations/20240806073726_drop_uniqueness_constraint_on_phone.up.sql

@@ -1,6 +1,16 @@
 alter table {{ index .Options "Namespace" }}.mfa_factors drop constraint if exists mfa_factors_phone_key;
 do $$
 begin
+    -- if both indexes exist, it means that the schema_migrations table was truncated and the migrations had to be rerun
+    if (
+        select count(*) = 2
+        from pg_indexes 
+        where indexname in ('unique_verified_phone_factor', 'unique_phone_factor_per_user')
+        and schemaname = '{{ index .Options "Namespace" }}'
+    ) then
+        execute 'drop index {{ index .Options "Namespace" }}.unique_verified_phone_factor';
+    end if;
+
     if exists (
          select 1
          from pg_indexes