Fix incorrect dtype precision upgrade for VectorIndex

[rly]

Motivation

Fix #630.

This is a critical bugfix, so I will release HDMF 2.5.8 after this PR is merged.

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running flake8 from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

[codecov]

Codecov Report

Merging #631 (f6d21d2) into dev (36db0e8) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev     #631      +/-   ##
==========================================
+ Coverage   86.13%   86.19%   +0.06%     
==========================================
  Files          41       41              
  Lines        8238     8239       +1     
  Branches     1776     1777       +1     
==========================================
+ Hits         7096     7102       +6     
+ Misses        811      807       -4     
+ Partials      331      330       -1     

Impacted Files	Coverage Δ
src/hdmf/common/table.py	84.13% <100.00%> (+0.77%)	⬆️

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