[docs] improve UPDATING.md around 0.31's [now] optional deps

[mistercrunch]

No description provided.

[john-bodley]

@mistercrunch the thrift reference may be somewhat irrelevant after #7115.

[codecov-io]

Codecov Report

Merging #7117 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #7117   +/-   ##
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  Coverage   64.59%   64.59%           
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  Files         422      422           
  Lines       20587    20587           
  Branches     2251     2251           
=======================================
  Hits        13298    13298           
  Misses       7167     7167           
  Partials      122      122

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[mistercrunch]

Ok, leaving the thrift reference in there just in case it changes something for someone out there (maybe the version ranges that are accepted are different, depending, ...).