refactor: Remove strong dependency from feature flag enum to (de)serialise tenant object (#40754)

## Description
> [!TIP]  
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> _Please also include relevant motivation and context. List any
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Fixes #`Issue Number`  
_or_  
Fixes `Issue URL`
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> _If no issue exists, please create an issue first, and check with the
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/test Sanity

### 🔍 Cypress test results
<!-- This is an auto-generated comment: Cypress test results  -->
> [!TIP]
> 🟢 🟢 🟢 All cypress tests have passed! 🎉 🎉 🎉
> Workflow run:
<https://github.com/appsmithorg/appsmith/actions/runs/15253157855>
> Commit: edc1653a9ec6e7dc3bd4411c7fdef74856bb3b9e
> <a
href="https://internal.appsmith.com/app/cypress-dashboard/rundetails-65890b3c81d7400d08fa9ee5?branch=master&workflowId=15253157855&attempt=1"
target="_blank">Cypress dashboard</a>.
> Tags: `@tag.Sanity`
> Spec:
> <hr>Mon, 26 May 2025 12:27:15 UTC
<!-- end of auto-generated comment: Cypress test results  -->


## Communication
Should the DevRel and Marketing teams inform users about this change?
- [ ] Yes
- [ ] No


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **Refactor**
- Updated internal handling of feature migration tracking for improved
flexibility. No visible impact to end-users.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

app/server/appsmith-server/src/main/java/com/appsmith/server/domains/ce/TenantConfigurationCE.java

@@ -1,6 +1,5 @@
 package com.appsmith.server.domains.ce;
 
-import com.appsmith.external.enums.FeatureFlagEnum;
 import com.appsmith.server.constants.FeatureMigrationType;
 import com.appsmith.server.constants.LicensePlan;
 import com.appsmith.server.constants.MigrationStatus;
@@ -47,7 +46,7 @@ public class TenantConfigurationCE implements Serializable {
     // the feature flags. This can happen for 2 reasons:
     // 1. The license plan changes
     // 2. Because of grandfathering via cron where tenant level feature flags are fetched
-    Map<FeatureFlagEnum, FeatureMigrationType> featuresWithPendingMigration;
+    Map<String, FeatureMigrationType> featuresWithPendingMigration;
 
     // This variable is used to indicate if the server needs to be restarted after the migration based on feature flags
     // is complete.