chore: refine vacuum (backport PR #16743 to release/v1.2.636-rc3)

[dantengsky]

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Summary

backport PR #16743 to release/v1.2.636-rc3.

If only orphans left, the vacuum should continue working as expected (purge all the orphans).

Instead of using prefix extracted from the "path" of object belong to the GC root, using block/bloom_filter_index/segment prefix directly.

Different from PR #16743 (which is being merged into main branch):

1. the block name is (kept as) prefixed with 'g' in this PR, as PR fix: add compatibility for reading non-standard decimal256 format #16585 has not been merged into main yet (or will not be).

   https://github.com/databendlabs/databend/pull/16744/files#diff-9d4aa60a0f7398a38f14b3c0ba1009a992cd2cd5b65f1e00642f5f1ba5d430d8R109

related unit test has been tweaked accordingly.

2. Logic test "tests/suites/0_stateless/18_rbac/18_0011_alter_own_username" also tweaked to align with the main branch

Tests

• Unit Test
• Logic Test
• Benchmark Test
• No Test - Explain why

Type of change

• Bug Fix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that could cause existing functionality not to work as expected)
• Documentation Update
• Refactoring
• Performance Improvement
• Other (please describe):

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This change is 

[dantengsky]

@TCeason PTAL :

logic test tests/suites/0_stateless/18_rbac/18_0011_alter_own_username has been updated to align with the version in the main branch.

[TCeason]

Alter user's modify is LGTM