lint duplicate migration numbers before merge and in `main`

Exhibit A: https://github.com/coder/coder/pull/15530

coder/coder#15448 was merged after coder/coder#15502, and both contained the same migration number. This passed all checks, and when we auto-deployed `main` to our dogfood environment we encountered a fatal error on startup:

```
Encountered an error running "coder server", see "coder server --help" for more information
error: connect to postgres:
github.com/coder/coder/v2/cli.(*RootCmd).Server.func2
/home/runner/work/coder/coder/cli/server.go:704
- connect to postgres:
github.com/coder/coder/v2/cli.getPostgresDB
/home/runner/work/coder/coder/cli/server.go:2625
- migrate up:
/home/runner/work/coder/coder/cli/server.go:2206
- migrate setup:
/home/runner/work/coder/coder/coderd/database/migrations/migrate.go:116
- create iofs:
/home/runner/work/coder/coder/coderd/database/migrations/migrate.go:82
- failed to init driver with path .: duplicate migration file: 000274_rename_user_link_claims.down.sql
```