Add a note on QueryCacheHits and QueryCacheMisses in the release notes

[frouioui]

Description

This PR modifies the v20 release notes to mention that QueryCacheXX does not refer to an actual query cache but rather to a query plan cache.

The metrics will be renamed in v21 through #16289.

Let's backport this to v20 for the release notes.

Related Issue(s)

• Follow up of queryserving, observability: instrument vttablet query cache plan hits/misses #14947

[vitess-bot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.70%. Comparing base (bb76046) to head (4ec48d3).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16299      +/-   ##
==========================================
- Coverage   68.71%   68.70%   -0.01%     
==========================================
  Files        1544     1544              
  Lines      198011   198011              
==========================================
- Hits       136064   136051      -13     
- Misses      61947    61960      +13     

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