Use admin request check for ReplicationDiff handler

[vadmeste]

Description

Instead of the S3 request validation, use admin check. This is to avoid issues
when validating requests issued mc using madmin library.

Motivation and Context

Fix mc replicate diff command when the server has an explicite region

How to test this PR?

1. Start two MinIO servers
2. Enable site replication
3. Create a bucket
4. mc replicate diff /

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Optimization (provides speedup with no functional changes)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• Fixes a regression (If yes, please add commit-id or PR # here)
• Unit tests added/updated
• Internal documentation updated
• Create a documentation update request here

[minio-trusted]

Mint Automation

Test	Result
mint-large-bucket.sh	✔️
mint-fs.sh	✔️
mint-gateway-s3.sh	✔️
mint-erasure.sh	✔️
mint-dist-erasure.sh	✔️
mint-compress-encrypt-dist-erasure.sh	✔️
mint-pools.sh	✔️
Deleting image on docker hub
Deleting image locally