avoid decommissioning DEL markers with single versions

[harshavardhana]

Description

avoid decommissioning DEL markers with single versions

Motivation and Context

ensure that we can decommission all versions including
null markers with other versions from the bucket.

How to test this PR?

Start locally with two pool setup

minio server /tmp/xl{1...4} /tmp/xl{5...8}

mc mb myminio/testbucket/

mc cp /etc/hosts myminio/testbucket/

mc version enable myminio/testbucket

mc cp /etc/hosts myminio/testbucket/

mc cp /etc/hosts myminio/testbucket/

mc cp /etc/hosts myminio/testbucket/

mc version suspend myminio/testbucket/

mc rm myminio/testbucket/hosts

mc ls --versions myminio/testbucket/
[2023-05-24 22:45:56 PDT]     0B STANDARD null v4 DEL hosts
[2023-05-24 22:45:50 PDT]   226B STANDARD 77a02c6b-1f02-4d98-8ee6-3eedd3d8f250 v3 PUT hosts
[2023-05-24 22:45:49 PDT]   226B STANDARD 3c7f4afe-dc42-41ec-8acd-3d3db423cef9 v2 PUT hosts
[2023-05-24 22:45:49 PDT]   226B STANDARD ba405e3f-3d3e-4ba8-bfaf-f41f2bda87f5 v1 PUT hosts

mc admin decom start /tmp/xl{5...8}

Once the decom starts, it must finish and the final result after the pool removal must be

[2023-05-24 22:45:56 PDT]     0B STANDARD null v4 DEL hosts
[2023-05-24 22:45:50 PDT]   226B STANDARD 77a02c6b-1f02-4d98-8ee6-3eedd3d8f250 v3 PUT hosts
[2023-05-24 22:45:49 PDT]   226B STANDARD 3c7f4afe-dc42-41ec-8acd-3d3db423cef9 v2 PUT hosts
[2023-05-24 22:45:49 PDT]   226B STANDARD ba405e3f-3d3e-4ba8-bfaf-f41f2bda87f5 v1 PUT hosts

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

[vadmeste]

LGTM

[ravindk89]

@harshavardhana our original Decomm PR had this already - is this a pure bug fix only?