Maintained postgres DB migrations state

- due to issue #813 (with mysql) we have rolled back all DB related changes:
030302a
873bc6d
34afece
which we previously released.

- since theere is no issue with postgres, it safer to NOT remove
the already-released DB migrations so that various consumers's
postgres DB states do not diverge. So this commit brings back
all those already-release postgres DB migration.

applications/credhub-api/src/main/resources/db/migration/postgres/V60__add_lowercase_credential_name_column.sql

@@ -0,0 +1,2 @@
+ALTER TABLE credential
+    ADD COLUMN name_lowercase VARCHAR(1024) GENERATED ALWAYS AS (lower(name)) STORED;

applications/credhub-api/src/main/resources/db/migration/postgres/V61__index_lowercase_credential_name.sql

@@ -0,0 +1,2 @@
+CREATE INDEX CONCURRENTLY IF NOT EXISTS credential_name_lowercase
+    ON credential(name_lowercase);

applications/credhub-api/src/main/resources/db/migration/postgres/V62__drop_index_secret_name_unique.sql

@@ -0,0 +1 @@
+DROP INDEX CONCURRENTLY IF EXISTS secret_name_unique;