Use half of the available cores when in watchAll mode

[brapifra]

Summary

Fixes #9116

Test plan

Added a new condition in the Returns based on the number of cpus test of the packages/jest-config/src/__tests__/getMaxWorkers.test.ts file

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[SimenB]

Thanks! Could you update the changelog as well?

[brapifra]

Done @SimenB 😄

[codecov-io]

Codecov Report

Merging #9117 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #9117      +/-   ##
==========================================
+ Coverage   64.73%   64.73%   +<.01%     
==========================================
  Files         277      277              
  Lines       11707    11709       +2     
  Branches     2875     2875              
==========================================
+ Hits         7578     7580       +2     
  Misses       3512     3512              
  Partials      617      617

Impacted Files	Coverage Δ
packages/jest-config/src/getMaxWorkers.ts	100% <100%> (ø)	⬆️

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[github-actions]

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