Fix outdated variable naming and comments to alive bitset

[k-yomo]

Some of the variables and comments for alive biset are still representing and describing old delete bitset.
This PR fixes those and also expired link.

[codecov-commenter]

Codecov Report

Merging #1387 (e61dd42) into main (328bd96) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1387      +/-   ##
==========================================
- Coverage   94.30%   94.29%   -0.01%     
==========================================
  Files         236      236              
  Lines       43088    43088              
==========================================
- Hits        40632    40630       -2     
- Misses       2456     2458       +2     

Impacted Files	Coverage Δ
src/core/segment_component.rs	100.00% <ø> (ø)
src/core/segment_reader.rs	91.16% <100.00%> (ø)
src/indexer/index_writer.rs	96.74% <100.00%> (ø)
src/fastfield/serializer/mod.rs	91.30% <0.00%> (-1.45%)	⬇️
src/fastfield/reader.rs	89.07% <0.00%> (-0.85%)	⬇️
src/directory/watch_event_router.rs	95.79% <0.00%> (-0.85%)	⬇️
src/aggregation/agg_req_with_accessor.rs	94.48% <0.00%> (+0.78%)	⬆️
.../termdict/sstable_termdict/sstable/block_reader.rs	82.14% <0.00%> (+1.78%)	⬆️

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[fulmicoton]

Thank you @k-yomo