Bump version to v3.1.8

[github-actions]

No description provided.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.2%. Comparing base (9873bc4) to head (d9d7a9b).

Additional details and impacted files

@@            Coverage Diff            @@
##             v3.1   #10107     +/-   ##
=========================================
- Coverage    83.2%    83.2%   -0.1%     
=========================================
  Files         865      865             
  Lines      376465   376465             
=========================================
- Hits       313498   313488     -10     
- Misses      62967    62977     +10     

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[levsha]

All changes are v3.1.7 -> v3.18 changes only:

# curl --silent https://patch-diff.githubusercontent.com/raw/anza-xyz/agave/pull/10107.diff | grep '^-[^+].*3\.1\.7' | wc -l
     640
# curl --silent https://patch-diff.githubusercontent.com/raw/anza-xyz/agave/pull/10107.diff | grep '^+[^+].*3\.1\.8' | wc -l
     640
# curl --silent https://patch-diff.githubusercontent.com/raw/anza-xyz/agave/pull/10107.diff | grep '^[+-][^+-]' | grep -v '[^+].*3\.1\.[78]'