chore(modules-sdk): db migration concurrency as envvar (#13965)

## Summary

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## Examples

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```ts
// Example usage
```

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## Checklist

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- [ ] I have added a **changeset** for this PR
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## Additional Context

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Author: carlos-r-l-rodrigues

.changeset/tame-candles-jump.md

@@ -0,0 +1,5 @@
+---
+"@medusajs/modules-sdk": patch
+---
+
+chore(modules-sdk): db migration concurrency as env var

packages/core/modules-sdk/src/medusa-app.ts

@@ -558,9 +558,10 @@ async function MedusaApp_({
       }
     }
 
+    const concurrency = parseInt(process.env.DB_MIGRATION_CONCURRENCY ?? "1")
     await executeWithConcurrency(
       moduleResolutions.map((a) => () => run(a)),
-      8 // parallel migrations
+      concurrency
     )
   }