fs: Update object modtime when metadata is updated

[vadmeste]

Description

Follow S3 behavior of updating object modtime when object metadata is
updated.

Motivation and Context

Fixes #12777

How to test this PR?

aws --endpoint-url http://localhost:9000 --profile minio s3 cp s3://testbucket/testobject s3://testbucket/testobject --metadata-directive=REPLACE should update object modtime

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)
• Documentation updated
• Unit tests added/updated

[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-zoned.sh	✔️
mint-gateway-nas.sh	✔️
mint-compress-encrypt-dist-erasure.sh	✔️
Deleting image on docker hub
Deleting image locally