fix(Migrator): use lowercase current_schema() for PG14 compatibility

[feiguoL]

Motivation

PostgreSQL 14 raises syntax error at or near "CURRENT_SCHEMA" (SQLSTATE 42601) when the migrator executes DROP INDEX and other DDL that references CurrentSchema().

The current code returns clause.Expr{SQL: "CURRENT_SCHEMA()"}. CURRENT_SCHEMA (uppercase) is a reserved keyword in PostgreSQL; PG14 parses the keyword first, then sees () as a syntax error. PG15+ tolerates it, but PG14 does not.

This broke the gorm CI matrix (postgres:14, oldstable) for every recent PR — see e.g. go-gorm/gorm#7837, go-gorm/gorm#7839, go-gorm/gorm#7840 — because tests_all.sh runs go get -u -t ./... and picks up v1.6.2.

Fix

Use the lowercase function form current_schema(), which is valid across all supported PostgreSQL versions (13+).

- return clause.Expr{SQL: "CURRENT_SCHEMA()"}, table
+ return clause.Expr{SQL: "current_schema()"}, table

Verification

• go build ./... — OK
• go vet ./... — OK
• go test ./... — passes

The gorm postgres:14, oldstable job is expected to turn green once this lands and a new tag is released.