cleanup(blockstore): remove migration fallback for Index

[kskalski]

Problem

#8459 enabled migration to CompletedDataIndexesV2 (contained in SlotMetaV2) and ShredIndexV2 (contained in IndexV2) which switched to serializing indices as fixed size bit-vec instead of using btree-set (which on the wire generates shorter integer lists).
Post 4.0 we don't need the old btree-set code that generates old format, it's enough that current bit-vec can understand it (that part could also be removed post 4.1).

This means CompletedDataIndexesV1, ShredIndexV2 and related structs can be removed.

This PR also removes the fallback deserialization code for supporting migrations of blockstore_meta::Index and blockstore_meta::SlotMeta columns - we could potentially keep it, but I think it's uncertain if any future migration could re-use it as-is, so for the sake of simplification it's better to cut it and re-create if we ever need it.

Summary of Changes

• remove IndexV1, ShredIndexV1, CompletedDataIndexesV1, SlotMetaV1
• rename CompletedDataIndexesV2 to CompletedDataIndexes, SlotMetaV2 to SlotMeta, ShredIndexV2 to ShredIndex, IndexV2 to Index
• still keep custom BitVec deserialization that supports data emitted by CompletedDataIndexesV1
• remove fallback deserialization that checked two formats

[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 83.1%. Comparing base (f41a763) to head (9091b48).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #11152    +/-   ##
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  Coverage    83.1%    83.1%            
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  Files         837      837            
  Lines      316652   317070   +418     
========================================
+ Hits       263170   263615   +445     
+ Misses      53482    53455    -27     

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